Cast Calculus

One of the first choices any writer has to make is how many protagonists 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. Lone protagonists are 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. While 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 Man vs. Man 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 it's kind of a "Duh" statement, read on.
 * Common genres or stories: These protagonist can be in any story and are unweighed by a large cast, and so they can 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 the typical drama with lots of long term character interaction, or a "stable" environment, these heroes will live and work in flux.
 * Signature series: Kung Fu, The Incredible Hulk, Metroid (Prime)

From there the duos are an even split between two traits: Body and mind, and temperament. One will be The Big Guy to the other's smarts. And then you have one emotional character versus a colder one. 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. If this sexual tension is resolved, then you have a Battle Couple (cue the shipping). Or for the more traditional dynamic, you have The Hero and their Sidekick or Love Interest. These duos are different in that the hero often serves as a mentor to the sidekick and protector to the Love Interest, though it's unlikely for the sidekick to graduate the role.
 * Common genres or stories: Again, any; 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.
 * Signature series: The X-Files, Supernatural, Hercules: The Legendary Journeys, Doctor Who (2005+) Aubrey-Maturin

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. The division between physical and mental doesn't get graded, but augmented with "social", a character who acts as a personable "face" for the group to balance the previous pair. If combat is involved, you get the Mighty Glacier, Jack of All Stats, and Fragile Speedster. Interestingly, from Trio on down you start seeing above archetypes merge into things like Genius Bruiser. 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 between each other and 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!
 * Signature series: Star Trek: The Original Series, Harry Potter, A Series of Unfortunate Events

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. Or, viewed another way, the Kirk is choleric, and the detached nature of the Spock and the raw emotion of the McCoy split and combine into an introspective and spiritual temperament. 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. It usually made up of a core Power Trio and one character to act as observer. Physically, the big-medium-fast division actually becomes big-medium-fast-frail. Frail characters are actually hearkening way back to the Duo's The Smart Guy, adding a character who sacrifices muscle for mind.
 * Common genres or stories: Drama is the order of the day for the quartet, though not always among itself. 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.
 * Signature series: Buffy the Vampire Slayer, Fantastic Four, Stargate SG-1, Sliders

The Five-Man Band, Six-Student Clique and The Magnificent Seven Samurai all vary some in their makeup, with each being "upgradeable" with Sixth Ranger. However; they all have the base Five-Man Band structure with one or more additions from the "auxiliary" list, like Plucky Comic Relief or Tagalong Kid, much like a core Three Plus Two cast of characters results in a Five-Man Band. Another one is that The Hero stops being a label and becomes a physically distinct character type that leads the ensemble's members. The personality and physical traits by this point can be pretty much mix and matched without worrying about maintaining a "balance" in the cast. Five is also the most notable of all of them for two reasons - one, it's one of the Tropes of Legend, and two, it's the biggest ensemble you can have without things getting convoluted - in all types of ensemble, combat, role and temperament. In combat, it will probably be Black and White Magic, with a balanced character, and a "light" character (Fragile Speedster) and a "heavy" character (Mighty Glacier).
 * Common genres or stories: Though roving bands of extended casts are not unheard of, they will take 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), Saved by the Bell (6), Seven Samurai & The Magnificent Seven (7), and Firefly (9)

Beyond seven, there are no fast and loose 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.

See also How to Gather Characters.

See also:
 * One: The Drifter
 * Two: Sensitive Guy and Manly Man
 * Three: Power Trio
 * Comic Trio
 * Freudian Trio
 * Three Amigos
 * Two Guys and a Girl
 * With a Friend and a Stranger
 * Four
 * Four Philosophy Ensemble
 * Four-Temperament Ensemble
 * Five: Five-Man Band
 * Four-Temperament Ensemble + Supine II
 * Four Philosophy Ensemble + The Conflicted
 * Six: Six-Student Clique
 * Five-Man Band plus Sixth Ranger
 * Seven: Magnificent Seven
 * Lots: Loads and Loads of Characters
 * Cast Herd

There's also a set of the above for all girl casts:
 * Two
 * Lovely Angels
 * Tomboy and Girly Girl
 * Three
 * Blonde, Brunette, Redhead
 * The Hecate Sisters
 * The Three Faces of Eve
 * Four: Four-Girl Ensemble
 * More: Amazon Brigade

And we also have an evil version of some of the above:
 * Two: Evil Duo
 * Those Two Bad Guys
 * Three: Terrible Trio
 * Four: Four Is Death
 * Five: Five-Bad Band
 * The Psycho Rangers
 * Six or Seven: Five-Bad Band + Enigmatic Minion and/or Morality Pet