It varies. Me and my bro used to play when his friends couldn't come over, so we had 2 characters each totalling 4 for a basic party.
Then, theres this new campaign, HUGE one.
8 players, 3 of which had leadership.
First off, this wasn't a normal game, as we are all the children of gods, I was Bane
, then 3 players got some Cohorts. I got a nightmare, a friend got a slaad, and the other got a fire elemental of large size. We are all 21st-25th level mind you.
So a total of 11 characters, 3 of which had only a couple levels in classes (the nightmare is a barbarian, and can talk, so I named him Mr. Ed.)
Now were about ready to start a normal campaign, 4 characters, and that's it.