I think the size of the team (or # of characters you use) depends on what your story is about. Every story has a "quest" , Ie: the Weather dominator; seeking the three elements, Serpentor; seeking the bodies to create him, Destro having nanites; how to stop him. Sometimes Your "Quest" is the main charatcer and the real Charatcers support that. Because I know the Characters, I find these the best stories where we don't get weighed down with personal stuff too much and we get to see the characters in their element doing what they need to do. (But I'm in the minority!)

Another problem is developing a character may require a flashback into a time when they were a civilian, and even though I'm trying to get more "civilians" (Corps! Cops, firemen, Construction workers and Wow! Power Girls) it's hard (and expensive!) to get a group of believable civilians and film a non-war scene. If you don't do that, you're forced to do an "Explanation" where a Character tells about the incident, which can lead to scenes of long boring dialog.

I think if you're getting in depth, use no more than 3 Characters. If you're 'just' getting into their characters, 5-8 is ok. If their cannon fodder Ala the movie "Hamburger Hill" throw the whole collection on! And enemies...go for broke....in Action sequences, you need tons of body count! But remember, this is MY OPINION. A lot of people here will disagree with me, but take a little from everyone and find what works for you!!