It's really annoying with this game. =\ Well, if you got with the ratio of 4 townies to 1 mafia, then 12 townies, 3 mafia and 1 serial killer could work well.
Just a quick note about Mafia, I reckomend aiming for a smaller amount of players, maybe 21 will do, maybe even lower. It seems that a lot of the Mafia games about that amount have lost everyone's interest after about a month.