One of the first choices any writer has to make is how many characters will lead the narrative. Believe it or not, that number matters. Too many, and you can barely get attached to anyone, just one and you'll never believe the author would kill them off.
So, which is the lucky number for Ensembles?
Let's start at one and work our way up. A protagonist is not some embryonic proto-cast that contains the traits of all Ensembles past and present, but rather has complete freedom to be whoever is needed for the story. Let's repeat that: lone. Though a one-man hero doesn't have to be an antisocial loner, they are very independent no matter what kind of character they are. Even the wimpy Action Survivor is at least able to survive. Interestingly, the best lone heroes make up for a lack of permanent cast with a varied supporting (though temporary) cast and (hopefully) some internal struggles to add depth. The Person vs Person type of dramatic struggle is common for the lone hero. Needless to say they are also invariably The Hero (well, let's say protagonist to hedge our bets). Though that's kind of a "Duh" statement, read on.
- Common genres or stories: These characters can be in any story and aren't weighed down by a large cast; they can realistically be Walking the Earth as The Drifter. Even if sedentary, they'll likely play the lone Action Hero against overwhelming opposition. What you won't see is either drama with lots of long term character interaction or a stable, unchanging environment. These heroes will live and work in a fluctuating world.
From there, the duos are an even split between two traits in terms of body, mind, or temperament (usually all three). One is the brawn to the other's brains; one is emotional and fiery while the other is more cold and analytical; one is by-the-book while the other feels rules should be flexible. The duo implies a certain level of equality: it's entirely possible for both to "share the billing" and be equal heroes. They'll likely be Heterosexual Life-Partners, but if they happen to be different genders, it's practically a law there'll eventually be Unresolved Sexual Tension (unless, of course, they are a Brother-Sister Team, and even that doesn't stop some writers). If this sexual tension is resolved, then you have a Battle Couple (cue the shipping). When the equality goes away, you have a different dynamic, The Hero and their Side Kick or Love Interest. These duos are different in that the hero often serves as a mentor to the sidekick or protector to the Love Interest. It's unlikely for the sidekick to graduate from the role to a true equal.
- Common genres or stories: Again, any and all. However, duos gain a certain level of stability as compared to lone heroes. The character interaction between them will often become deep and nuanced to a degree not often seen with other ensemble numbers. Duos are likely to be in Action-Adventure shows, possibly playing Detective or fighting crime.
When you get to Power Trios the different splits get more interesting. The personalities divide into three, not so much dividing the Red and Blue Oni as creating a "balance" personality wholecloth. Note that any of them can be the lead hero. If the division is between physical and mental, it doesn't get degraded, but augmented with a balanced character, a character to mediate the previous pair. In personality and behavior terms, it may involve a scale of Nice Mean And In Between. If combat is involved, you get the Mighty Glacier, Jack-of-All-Stats, and a Fragile Speedster and/or Glass Cannon, or perhaps a Fighter, Mage, Thief setup. Interestingly, from Trio on down you start seeing the above archetypes merge into things like Genius Bruiser. They may or may not be a Freudian Trio of The Kirk, The Spock, and The McCoy. It's worth noting that from here on out a girl being in the group gets logistically easier and much more common.
- Common genres or stories: A trio is downright homey, and not in the sedentary sense. Three is the number where a family of friends can be born; characters can become True Companions. Even if they don't see each other as a family, the dynamics between them will give viewers a sort of "safety net". Past this size, even when the group's adventures lead to them traveling the world (or galaxy), they will tend to work out of a base (or Cool Ship) which often becomes something of a character in its own right. Trios work best in genres where there's room to interact both with each other and with the environment, from here on down an ensemble can hypothetically devote an entire episode or chapter just to the cast interacting. These guys are likely to be in an Action-Adventure or Drama. Or both!
The quartet is a challenge: just enough people for things to get convoluted, but not enough to lose track of anyone. The Four-Temperament Ensemble divides the Red and Blue Oni in half again: the Red Oni splits into sanguine and choleric, and the Blue Oni into phlegmatic and melancholic. Alternatively, the characters can be split into a Four-Philosophy Ensemble in which the characters have different viewpoints and philosophies, rather than personalities, which interact as they face problems and have to reconcile their differences to come to agreement.
- Common genres or stories: Drama is the order of the day for the quartet, particularly internal drama. A quartet is likely to 'split up' in a given episode, giving each a chance to play off not just each other but dealing with the various aspects of the plot and the week's guest characters. Expect occasional reminders of why everyone is special and important, even if they're Muggles.
Five and anything beyond that point tend to vary quite widely in makeup, as the personality and physical traits by this point can be pretty arbitrarily mixed and matched without worrying about maintaining a "balance" in the cast. Typically, The Hero stops being a label and becomes a physically distinct character type that leads the ensemble's members. It's quite common for a group of five to consist of two trios (and an optional extra), often based on gender - these may or may not conform to the Three Faces Of Adam and the Three Faces of Eve.
- Common genres or stories: Though roving bands of extended casts are not unheard of, they will carry their home with them, be it a space ship, a Mystery Machine, or merely the clothes on their back. These enormous ensembles practically write a Drama themselves, never mind having Hilarity Ensue due to outside events.
- Signature series: Power Rangers (5), How I Met Your Mother (5), Friends (6), Saved by the Bell (6 — 7 if you count Mr. Belding), Startrek The Next Generation and onward (7+), Seven Samurai & The Magnificent Seven (7), Justice League (usually 7), Digimon Adventure (7, later 8), Arrested Development (9) and Firefly (9)
Beyond five, there are no hard and fast rules for the cast as a whole. However, even with Loads and Loads of Characters, the cast members can be broken down into a Geodesic Cast or a set of Cast Herds, each iteration of which usually follows one of the archetypes listed above. Individual characters may belong to a single group only, or they may belong to several, with their role sometimes changing depending on which group they're interacting with.
This list primarily applies to gaming and action/adventure tales:
- Seven: The Magnificent Seven Samurai
- Eight: Personality Blood Types, can be four Rh+ and four Rh-
- Lots: Loads and Loads of Characters
There's also a set of the above for all girl casts:
- Four: Four-Girl Ensemble
- More: Amazon Brigade
And we also have an evil version of some of the above:
- Two: Evil Duo
- Three: Terrible Trio
- Four: Four Is Death
- Five: Five-Bad Band
- Six or Seven: Five-Bad Band + Enigmatic Minion and/or Morality Pet