I've run 1 PC sessions and 10 PC campaigns, but I have to say that beyond 5 or so just makes my head hurt, and having less than 3 really takes a lot out of the game. Currently I have 3, which is great.
I've done games with everywhere from 1 to 6 players. I think I enjoy the larger groups more, and I'd probably be fine with more than 6 players, but that's the most I've ever had at one time.
I really like 5 or 6. It means that if Jim and Barb can't come this week, you can still game with no problems. When I played with only 3 or 4, invariably someone would cancel and sessions would go down the tubes.
I put 3-4-and 5. I currently have seven players in one of my games with an eighth wanting to join. It is great that I have an assistant dm. Otherwise it would be way too many for me to handle.
I did a group of 11 at a university club - never again
4-6 is the ideal - i like player-player interaction and since my normal group is a group i've known for a while its easy enough to keep them all involved