4 players is not enough, but in my opinion, over 16 is too many. If you get too many people it becomes chaotic and round-based games take too long to finish, leaving people who die early to sit around most of the time.
8-14 players is my ideal.
for fps i would say 12 would be ideal. I used to play a number of games competitvely and i could never stand playing anything other than 5v5 or 6v6 as the maps get too cramped.