"At least the World will end, an event anticipated with great joy by many. It will end very soon, but not in the year 2000, which has come and gone. From that I conclude that God Almighty is not heavily into Numerology."Since mankind began to count, they've wanted to assign importance to numbers. This might be due to the desire to assign significance to everything. This might be to make it easier to have common threads in stories. Maybe it's just to look cool. Regardless of reason, numbers are often given significance in stories, and mention of a number in a story can indicate a meaning beyond just counting out items. Equally often, people will interpret these numbers to see significance where none exists. Here are some of the more common motifs. See also Arc Number
— Eugene Debs Hartke, Hocus Pocus
0 - This number represents The Void, the complete absence of being. It's usually something menacing, and it's always a bad moment when there's nothing left. Characters named after the number 0 form the trope My Hero Zero — though they're as likely to be villainous. 1 - This is the number for The Hero, and it represents his standing alone as the best. 1 also represents the beginning, and the primal source of power. This is why a hero will succeed if there's only One Bullet Left or a Million to One Chance - 1 is just that powerful. The Ace, representing the number 1, is also typically the strongest card in standard poker. 2 - 2 represents duality - and thus, it stands in for The Villain. Sometimes, it's rather blatant (such as Two-Face in Batman), but other times it's more subtle. 2 is the prime source of moments where the villain says that he and the hero are Not So Different. Also, because 2 identical people represent the loss of individuality, 2 also pops up as Creepy Twins. Internally, a hero divided in 2 may have any number of varieties of an Enemy Within. Conversely, 2 represents the number of parties in any basic Conflict, be it with The Villain, parallel protagonists, former friends, Destiny, Star-Crossed Lovers, what-have you. Placing 2 images or characters side-by-side puts them in implied opposition. Of course, it is possible to feature 2 in a positive light: this generally shows up represented in a pair of characters who are very close, such as Bash Brothers or a Hero and Lancer duo. 2 can also represent a cosmic force of balance: think yin and yang, and the Balance Between Good and Evil. 3 - 3 is the most basic way to represent a structure, in a triangle, so it has representations of power. This is the origin of tropes such as the Rule of Three, the Power Trio, The Hecate Sisters and The Three Faces of Eve. Christianity believes that God originated this, having always been Father, Son, and Holy Spirit. A basic theme in Blue Man Group is that 3 is the fewest number of people needed to have an alienation or unpopularity. In most voting political systems, getting this also comes into play (majority is 2 out of every 3 votes). Otherwise known as 2 is company, 3 is a crowd. 4 - 4 actually has different meanings in different parts of the world. In European traditions, 4 represented the physical world, and the 4 elements that made up everything (fire, water, earth, and air). They also corresponded with the 4 cardinal directions (north, south, east, and west), and thus represent a grounding in reality and are generally positive. In Christian traditions, the number 4 symbolises the Horsemen of the Apocalypse, and is negative. Classically-based Elemental Powers use this pattern. See also Four-Temperament Ensemble. However, in Asian traditions, the number sounds almost like the word for "death" in all languages that borrow from Chinese, which arises in the trope Four Is Death. Curiously, the one aversion are The Four Gods, although they frequently are depicted as rather dangerous (and at least one is frequently depicted as explicitly evil, like Seiryu in Fushigi Yuugi). 5 - Also distinct in both Eastern and Western traditions, although not as radically different as they are with 4. In the Asian tradition, the physical world was tied with 5, and they classically had a 5-pronged elemental system (adding wood and metal to the 4 Europeans used while removing air). The relationship between them was much more complex, as was the Elemental Rock-Paper-Scissors associated with them. This balance directly led to the formation of Five Man Bands. In Europe, 5 also represented a 5th element, but there it represented Ether, or Quintessence, the mystical substance which made up spirits and the like. It thus represented the esoteric and the spiritual plane, and was considered completely detached from the physical realm. This is most obvious in pentagrams.* Some people holding to the idea of 4 distinct personality types also hold the idea of a 5th type, balanced between the extremes of the 4. It either has all their strengths (superior, above), none of their strengths or weaknesses (equal, between), or none of their strengths (inferior, empty). In some Japanese systems including that of Musashi's Book of 5 Rings, the 5th element is void, with associations similar to Western quintessence but a strong Zen flavor. "5" in Japanese is said as "go", which can lead to a pun. See One, Two, Three, Four, Go!. 6 - Most of 6's meanings have been swallowed by the fact that the number 666 is supposed to be the Number of the Beast. From a mathematical standpoint, 6 is an important number, since it is the first number that is divisible by both 2 and 3, and thus may be used to represent stability and order (see 12, below). 6 is the smallest perfect number, which is of little practical importance, but by the same equality of 1+2+3=6, and also 1x2x3=6 also the third triangular number. Curiously, 6 is also the first number for which several mathematical constructions break down, due to being the smallest number that is not a power of a prime. 6 is the atomic number of carbon, and thus irreplaceable for life as we know it. The fundamental molecules of life are furthermore made up from only 6 elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur.note Still, the imagery at this point invariably depicts 6 as a negative number, and only evil characters attach themselves to it. 6 is considered evil in the Bible because it falls short of 7, the perfect number. This is taken even further in German, where it is pronounced very similar to sex (as in intercourse), also adding a dirty connotation (and source for double entendres). In Swedish the two words are pronounced and indeed spelled the same, leading to many a juvenile joke. Interestingly enough, 666 is considered a good number in Chinese culture. For example, on June 6th 2006, there were many Chinese marriages, whereas most Westerners would try to avoid this. 7 - 7 is the most popular lucky number, and was originally the number of the holy virtues man was said to have. Of course, their opposites are much more memorable, but 7 is still overall a positive force, to the point that days of the week are still numbered in 7. Of course, biblically, 7 can also be the number of finality: the 7 hills of Rome, the 7 angels and the 7 seals, etc. There are Seven Heavenly Virtues that oppose Seven Deadly Sins. In Japanese culture this manifests in The Seven Mysteries. Also, in the Japanese language, Seven Is Nana. See also The Magnificent Seven Samurai, the Ensemble that falls under this number. In Christianity (and in Judaism before it), 7 is the number of perfection, and thus the number of God. For songwriters, "seven" is a convenient rhyme for "heaven", and is also useful because it scans as two syllables. Plot Coupons also have a tendency to come in 7s, especially in Video Games. 7 also comes up as a limit on human minds - people are said to be able to remember only 7 different numbers in a single trial. Similarly, the Incident Command System for dealing with emergencies forbids each person from giving direct orders to more than 7 people. 7 also comes up in gambling, specifically the roll of 2 dice, in which the most common resultant sum is 7. This helped cement its place as a "lucky" number. 7 is often seen as a magical number, and 7th sons of 7th sons were believed to have magic powers. 8 - 8 typically has notions of the hidden and esoteric, in part because it looks like an infinity symbol on its side (which was lampshaded on The Tick). It doesn't come up much, but expect plenty of mysteries when it does. 8 is also the luckiest of lucky numbers if you are Chinese, so an office on the 8th floor at number 88 8th Avenue, for instance, would be considered to have excellent Feng Shui no matter how the furniture was arranged. Some Neo-Nazis use the number 88 as a code for the phrase "Heil Hitler," H being the eighth letter of the alphabet. A further group used to call themselves Combat 18, 18 here standing for AH, Adolf Hitler. 8 is an important number in chemistry, as the outermost valence shell of an atom (which is where almost everything chemical happens) can contain at most 8 electrons; covalent bonds between atoms in molecules allow them to share electrons, so that all manage to count 8 in their outermost shell. Something similar holds in nuclear physics, which is why oxygen (with 8 protons and 8 neutrons) is one of the most stable and therefore common elements. 8 also has both 2 and 4 as factors, so it's convenient for combining pairing of things with quadruplets of things—such as between the parts of Golden Sun, where the 2 halves together have 2 characters representing each of 4 elements. 8 is also an important number in the Shinto religion of Japan. It is said in the creation myth that Izanagi-no-Mikoto and Izanami-no-Mikoto had a group of 8 perfect children who became the islands of Japan. Another instance is in the belief that the kami (deities) represent all things, and are "countless", which in Japanese can be read as "8000000". In computing, bits are collected in groups of 8 to form bytes, and larger units of memory are made up from whole bytes. This is why the 8-bit systems were followed by 16-bit, 32-bit, and 64-bit systems, but note that the 24-bit level is not unheard of: some computer systems had a 24-bit address bus, and 24 bits is "millions of RGB colours". There's also a trend of Plot Coupons coming in groups of 8 in Video Games; most notably the 8 non-bonus worlds of pretty much every Mario game; as well as Mega Man's 8 Robot Masters. In music, the octave is the fundamental interval, and there are 88 keys on a piano. 9 - 9 is the apex of the single digits, and thus the apex of worldly power. Appearances of this are usually sources of great power, since it's essentially a trio of Power Trios. Very big in Norse Mythology, which may be why Tolkien, fan of the Vikings that he was, made 9 Ringwraiths (and 9 Walkers). The magic square of numbers 1 through 9 is sometimes an important symbol. Also, there are 9 orders of angels in Judaeo-Christian theology, and 9 layers of Heaven and Hell in Dante's Divine Comedy. Another facet of 9 is gestation, as in 9 months, and thus it is closely associated with childbirth and fruition. In Japanese culture, however, 9 is often considered a cursed numeral because the word for it, "ku", is pronounced the same as the Japanese word for pain and suffering. Despite this, it does have connotations of power as well. Kitsune, for instance, wield magical powers whose strength and power are indicated by how many tails they have, and the maximum number of tails a Kitsune can have are 9 tails (kyubi). 10 - Since most people are born with 10 fingers (which are the basis for beginning to count), 10 is a comfortable number to express, either via powers of it or the number itself. It's extraordinarily popular for grouping, to the point that people will try to make things fit into a group of 10 whether or not they'll fit.note The entire Persian/Arabic numeral system (the one most Westerners use, because it's better than the Roman system) is based entirely on the number 10, as is the metric system. Tropes spawned of this shoehorning include Exty Years from Now. In certain numerologies, 10 is also used as shorthand for 'lots'. 11 - With 10 as a maximum, 11 means going one step past the limit and breaking the rules. Conversely, thinking about it as the time on a clock has more or less the opposite meaning- "the 11th hour" is the last hour before midnight and typically involves getting things done just on time. Being a symmetrical number can also bear some sinister connotations (see 2, above). The German word for 11 is pronounced "elf", which is sometimes taken advantage of in works that like Bilingual Bonus or Gratuitous German. 12 - 1, 2, 3, 4, and 6 are all factors of this number, making it an early highly composite number. It symbolizes completeness as a result, and is easy to divide into smaller groups. It also has heavy mystic implications—for example, the 12 signs of the zodiac (Western and Eastern alike). It can be used for time symbology as well, as 12 o'clock is midnight and noon; the end and beginning of a day, or the apex of a day. It is also often the largest number for which a stand-alone word or symbol exist, and being so heavily associated with harmony and completeness, it's surprising no major culture has a base-12 counting system. This came about namely because of the civilizations not using the five digits on each hand to make 10, rather looking at one hand and using the thumb to count the segments on each of the remaining fingers. Being "a dozen", it signifies the largest easily comprehensible small number; anything over "a dozen" can be rounded down in common parlance. Most non-decimal measurement systems are divisible by 12 (pieces of eight being an exception). Big in The Bible. 12 tribes of Israel, 12 apostles, 12 days of Christmas (close enough), etc. This may have something to do with 3 (the Trinity) being multiplied by 4 (the four corners of creation). See 13 for further examples. Also notable in Greek Mythology; the Olympian gods form a set of 12 called the Dodekatheon (later imported to Rome as the Dii Consentes). The exact roster varies depending on which source you check, but 12 is a recurring theme for the principal gods. In western music, the octave is divided into 12 semitones, leading to the 12 major and 12 minor keys of music (Db Ab Eb Bb F C G D A E B F#). 13 - The number of full moons in a year, the number of people seated at the Last Supper, the traditional number of witches or Satanists in a coven, the age at which someone officially becomes a teenager etc. Bad medicine. Often not assigned to a house (it will be numbered 11A or 12A depending on the numbering system) or to a floor in a tower. The Eighties' Eagle comic strip The Thirteenth Floor featured a Holodeck run by the building's A.I. occupying the unlucky floor, seeing as how nobody wanted to live there. Also used to suggest certain words beginning with the letter M (the 13th letter of the alphabet). Also, the number of cards in a standard suit as well as the number of weeks in a quarter (e.g. fiscal quarter) of the year (52/4). 13 has a long history of being unlucky; the Greek pantheon kicked out Hestia, goddess of the hearth, to make room for Dionysus, god of wine, in their circle of 12; it was believed back then that having a 13th deity in the highest echelons of the pantheon would lead to ruin, and that alcohol was much more important on the sacred scale than the home. Judas Iscariot, the 13th attendee at the Last Supper, was the one who betrayed Jesus, and it was believed that Jesus's crucifixion happened on Friday the 13th — or if The Da Vinci Code is accurate, The Purge of the original Knight Templar occurred on that date. There is evidence of other cultures developing forms of triskaidekaphobia before even that. Conversely, many Wiccans and witches consider 13 a lucky number, most likely due to its historical associations with witchcraft. Note that 13 is one more than a dozen, and thus comprises "A Baker's Dozen", i.e. a little something extra. Whether this is an aversion or an invocation of 13's unluckiness (on the principle that someone will order no more than a dozen hot cross buns, Elite Mooks, what have you) is hard to say. (The most widely held belief behind the reason for 13 being a baker's dozen is that the 13th roll, loaf, what-have-you, is intended as a redundancy in case one of the other twelve were somehow defective) There were also 13 states at the time of the US Declaration of Independence, as commemorated by the 13 stripes on the flag. The $1 bill has several references to the number 13: 13 stripes on a shield, 13 stars, a branch with 13 leaves and 13 berries, a sheaf of 13 arrows, and the phrase "E Pluribus Unum" which contains 13 letters. (The fact that 13 constantly crops up in American symbolism has led numerous conspiracy loons to theories of ever-increasing inanity. They always overlook the simple explanation of 13 states.) 13 may also indicate a change in circumstances. The 13th member of a group of people may change them or bring them closer to some goal. (e.g. Roxas in the Kingdom Hearts series.) This is likely due to the meaning of the Death card in the Tarot, which is card number 13. Note: Misinterpretation of this meaning may also lead to 13 bringing about death. (e.g. in Doctor Who, The Doctor only has 12 regenerations.)
101 Dalmatians, A Thousand And One Nights, Million to One Chance, etc. A Year and a Day has the same role. 108 - see the entry. 666 - Number of the Beast. EVIL! (Wait, or was that 616? Or 216?) Prime numbers in general are popular for the number of Plot Coupons, especially 3 and 7. Any work that references numerology in general (or gematria, the Kabbalic study) will heavily reference this trope. See also Arc Number. More examples can be found at the 1kbwc Wiki
Here are some numbered tropes by number and type.