well that's an interesting and vague question.
Sports (active players only)
American football: 11 on each team
football (or soccer in the US): 11 on each team
baseball: 9 on each team
hockey: 6 on each team (5 + goalie)
basketball: 5 on each team (although "pick-up" games are commonly 2- or 3- man teams)
Depends on the game. Computer games depends upon the game's coders.
Console games typically support 4 per console or up to 16 online, though some games support more.
Anywhere from 2 to 8 or more depending upon the game.