the first time i played silver and the first time i played ruby my team consisted of my starter, the regional bird, and then for the elite four i threw together the "strongest" pokemon i had for back-up incase something happened to my started lol.
nowadays i normally have a full team, but it depends on the playthrough, the pokemon avaliable, and if i'm doing a challenge. there was one challenge that i went though the game with just four, and i believe there was one time i went through the game with five. i don't think i've done any with less than that, though, unless it was a solo run.