Arrow: At first Oliver Queen is barely able to defeat one member of the League of Shadows, but as the series continues he fights more and more League ninja at a time, and many of them are little more than mooks who are taken out by supporting characters.
Babylon 5: Presents a rare good-guy example of this: when there's only one White Star, it's unstoppable. Once there's a fleet, they start getting taken down by mid-level enemies, often with no Vorlon or Shadow support. This is especially bad since the White Stars are meant to be able to learn from each hit it takes, so that the armour gets stronger after every battle. Even as late as the fourth series, the White Stars continued to get weaker: in Series 3, it takes 3 White Stars to destroy a Shadow warship, but by the battle of Proxima 3, 4 White Stars are needed to deal with a single Earth destroyer, an incredibly simpler ship with far less firepower, which Sheridan stated was weaker than The White Star.
This also seems to apply to the Shadows and Vorlons - Shadow battlecrabs were notoriously difficult to kill and Vorlons were pretty much invincible. Until the Battle of Coriana, when they started blowing up left and right.
Marcus Cole explained to a group of thugs why they should tell him what he wanted to know: "Because if you don't, then in five minutes I'll be the only person at this table still standing. Five minutes after that, I'll be the only person in this room still standing. So, who's in?" After he makes good on this threat, he laments that, "Now I have to wait for someone to wake up."
Battlestar Galactica (2003): You see this near the end of the show. Where initially a few Cylon Centurions were nigh-unstoppable juggernauts that needed to have their heads blown up before they stopped, in a suitably dramatic Storm the Castle situation the dangerously outnumbered Battlestar crew can bring them down in droves with a few sporadically-fired 9mm rounds. Admittedly, they had the help of other Cylons by this point, which could mean better bullets. Plus the fact that the Colonials in the start of the series were armed for the last Cylon war, and the new units were massive upgrades, while at the end of the series they'd been fighting Cylons for years on end, and had plenty of time to improve.
Buffy the Vampire Slayer: Buffyverse vampires were particularly subject to this trope. Individual vampires could be fairly respectable opponents, though they still had a bad track record of getting one-stab killed after Season 1. Whenever vampires gathered in groups, they were cannon fodder. One just hopes they don't have problems with splinters. This is justified in that each character is constantly improving their fighting skills.
The final season mixed this trope with a good dose of, ahem, Villain Decay. The first Turok-Han 'uber-vamp' was a nearly unstoppable force very narrowly beaten by the Slayer after several victories. In the finale, however, the Scoobies went up against an army of them, and Xander, Anya, and the slayers-still-in-training were taking hundreds of them down easily. In the DVD commentary, Joss Whedon points out that this was a conscious decision, claiming that "they couldn't all be as hard to beat as the first one," since that would make the last fight unwinnable. No in-universe explanation is given, though each potential Slayer had presumably been trained by Buffy and had just each received powers equal to a fully fledged Slayer, so it isn't entirely implausible. Several Slayers die in the battle until Spike saves the day with a heroic sacrifice. Nothing else left to do but recite the Mantra and shrug it off.
Applies to Slayers too: Buffy on her own can take any number of vampires, but whenever she's fighting with Faith or Kendra, at least one of the Slayers gets into a position where they need the other's help.
Perfectly illustrated in Daredevil. In the first season, Daredevil is nearly killed in a fight with one Hand ninja, Nobu. In the second season, he faces the whole Hand organization, and initially has a hard time beating them. However, once Stick tells him to listen to the sound of their weapons, Daredevil is able to fight them. In the season finale, he and Elektra easily cut through dozens of them.
Doctor Who: The amount of danger presented by the Daleks seems to always be inversely proportional to the number of Daleks present. When the Doctor and company are only facing one, as in "Dalek", it's a potential end-of-the-world scenario. When he faces millions as in "The Parting of the Ways" and "Doomsday", all it takes is a quick Deus ex Machina to save the day. When he's back to three in "Evolution of the Daleks," it takes a betrayal of their enslaved army to take them down, and one still gets away. You can be sure that last one is once again going to be a serious threat when it reappears.
Referenced in "Doomsday", when Daleks and Cybermen declared hostilities:
Cyberman: We have five million Cybermen. How many are you? Dalek: Four! Cyberman: You would destroy the Cybermen with four Daleks? Dalek: We would destroy the Cybermen with one Dalek!
The Daleks do seem capable of making good on this threat — they are so much more advanced that during the ensuing fire fight they are seen to take out dozens of Cybermen, but not one of the four Daleks takes damage.
The Doctor himself makes heavy use of this trope. As Rose says in "Doomsday", "Five million Cybermen? Easy. One Doctor? Now you're scared." And in previous episodes, even in the old series, the Daleks eventually started a policy of dropping whatever they were doing and fixating entirely on the Doctor once they knew he was present.
The Series 4 Finale "Journey's End" and the Series 5 Finale "The Big Bang" seem to indicate that Millions of Daleks < One Doctor < 5 or Less Daleks < one very pissed-off River Song
This even applies to Davros, their non-Dalek creator. When he's surrounded by Daleks, he's an Axe-Crazy and rather unintelligent cartoon Hitler-clone who spends most of his time yelling at them. When there are no or few Daleks around, he's a much calmer and terrifyingly intelligent Manipulative Bastard.
In "Forest of the Dead", as the carnivorous shadow creatures approach, take pause, and then flee after this one line.
The Doctor: I'm the Doctor, and you're in the biggest library in the universe. (Beat) Look me up.
Game of Thrones: Jorah Mormont, Grey Worm and Daario Naharis take on upwards of fifty guards with barely a scratch off-screen during the sack of Yunkai.
In Grimm, a single Wesen criminal is a serious threat that can only be tackled by a Grimm or another Wesen, and even then it's chancy. When small armys of Wesen attack, Badass Normal Hank can usually punch out a couple with no trouble.
Kamen Rider Dragon Knight: The Mooks suffer from an extreme case of this. A group of them are nothing but cannon fodder for an unmorphed Len to kick around. One, on the other hand, once required two Riders to use some of their strongest attacks.
This is actually due to the source footage. In Kamen Rider Ryuki, we have a Monster of the Week called Gelnewt which was of standard monster strength and was fought over the course of a two-part episode. For Dragon Knight, the producers decided to turn the Gelnewt into the series' Mook, meaning this trope suddenly applies. Ironically, this filtered back to Japanese, where Gelnewts are the Mooks in the Den-O and DecadeCrossover movie entirely because there was a surplus of the suits left over from the filming of Dragon Knight.
Kamen Riders, to some extent. When Kit fought alone against Axe, Spear and Strike all at the same time, he manages to defeat the three of them and even finish Spear. But when he fights Axe solo, he loses both times and has to be bailed out by Wing Knight.
In Decade's first movie: two Riders vs. all of Dai-Shocker? The Riders were easily trounced. Even when the rest of the Riders arrive, they are still heavily outnumbered, but it was enough to turn the tides.
In Kamen Rider Kabuto, how hard it is to kill Salisworms depends on how many there are; just one or two may require a Finishing Move, in a larger group they can be killed with a few swipes of a weapon, and in a very large group just a hard punch can destroy them.
The 40th Anniversary movie, OOO, Den-O, All Riders: Let's Go Kamen Riders, a team of all main Riders from Ichigo to OOO, gets knocked around by Great Leader. However, when Ankh gives OOO the Shocker and Imagin medals to become TaMaShii Combo, Great Leader is dropped. Interestingly, when the Secondary Riders show up to help finish the fight, they do overcome Great Leader.
Legends of Tomorrow: Played straight in Season 2 Episode 3 With actual ninjas, and some samurais.
Leverage: Sees a lesser degree of this. Eliot can drop a crowd of mooks just by breathing hard (especially in a season premiere as an Establishing Character Moment for new viewers, even moreso if they have guns), but a one-on-one fight takes him a while and effort.
Revolution: Played totally straight when Miles fights off an entire militia in the pilot. As promised, he's "good at killing".
Satisfaction: A justification for the trope is given in this Russian period miniseries. A fencing instructor makes his student fight three of his servants at once. After the student loses the first round the instructor asks him why he lost. The student says that he was outnumbered. The instructor tells him that he is wrong, because their greater number is actually their weakness: none of them wants to get injured, each would prefer one of the other guys to be in harm's way, and hence none of them is willing to show some initiative and do something really daring. With that knowledge the student naturally proceeds to kick their asses.
Saturday Night Live: Hilariously lampshaded in an Adam Sandler-era SNL skit, where the group of ninjas do a review of what went wrong after another failed attack. "How did we say we were going to attack the guy?" "All at once..." "And how did we attack?" "One at a time..." Sandler's hooded ninja speaks up about the use of throwing stars, noting that they are not a good idea in a large group, then pulling back his hood to reveal one stuck in his forehead. The gang ends up deciding to get their confidence back by beating up the next person they meet in the lobby - who of course turns out to be Bruce Lee.
Scrubs: Had this in the fantasy scene where over a dozen asian interns attackers (wearing surgical masks much like ninja masks) are handled with ease by Turk and Todd. Admittedly it's a fantasy scene so no justification is neccesary but it still fits the rule.
Stargate-verse: Regularly seen. Stargate SG-1 has the justification that Tau'ri ships like the F-302 Mongoose and Daedalus-class battlecruiser are simply better-engineered than their opponents', which tend to be Awesome, but Impractical. Same goes for ground engagements: Numerically smaller Tau'ri forces mow through dozens, even hundreds of mooks at a time due in part to betterequipment and tactics.
This goes both ways, of course. The battle at the Ori supergate had four Ori ships curbstomp an entire fleet of Jaffa, Asgard, and Tau'ri ships.
Star Trek: This show is all over this trope. In fact this could be the very basis of their famous Redshirts. You see groups of Redshirts get vaporized, but Scotty survives into the 24th century! Even in the future, multiple Starfleet personnel get wasted during the course of TNG, but Worf makes it, despite The Worf Effect. Likewise, while whole armadas of ships get pummeled, single starships win the day. This even applies to the bad guys. A single Borg cube can cause so much havoc, yet every time we seen a bunch of Borg cubes, they're usually destroyed immediately after.
In Star Trek: The Next Generation episode "Best of Both Worlds, Part II," the Borg blows away an entire Federation taskforce, without even a fight. However the Enterprise-D is able to go it alone against the Borg cube, and escape with only minor damage. This is justified by A) the Borg having Picard, thus knowing what Starfleet would do and B) the ships moving in in single-file formation. When the Borg return to Earth in Star Trek: First Contact, Starfleet has learned their lesson and employ a "swarm" tactic, which leads to a severely damaged Borg Cube, even if many of the ships are damaged or destroyed.
Somewhat lampshaded in the TNG episode "Contagion" where Riker says "fate protects fools, little children, and ships named Enterprise."
Speaking of a single Borg-cube wiping out all of the attacking Starships, Conservation of Ninjitsu earns its Magnum Opus when the lone ship Voyager enters the Borg home-turf of the Delta Quadrant, turning the tables completely as it takes on the entire collective— apparently destroying it when Admiral Janeway kills the Borg Queen.
Also seen in Deep Space Nine during the Dominion war arc. The Dominion have powerful Jem-Hadar fighters/destroyers that attack in large fleets to overwhelm big slow clumsy ships. The Defiant is the first Federation ship built along these tactical lines, and its first combat against the Dominion sees it nearly destroyed by only two or three Jem-Hadar fighters. Later in the series, the Defiant and other Federation and Klingon ships are seen swatting fleets of them like flies.
Also, early seasons would sometimes feature battles with multiple resurrected monsters, who would usually go down with just one or two hits. Eventually subverted in the third season premiere where a villain and four resurrected monsters, all giant sized, tear the Megazord to pieces.
The Megazords are victims of this as well. A single combined one from every machine available can destroy practically anything, but two or three fighting together usually get knocked around like ragdolls.