I vote we keep it at 2 mafia games simultaneously at once.
I have no horse in the race, since I've sworn myself off of forum Mafia games, but I don't see the need to place a cap on the number of games. These posts no longer show up on the news feed, right? So the increased traffic doesn't interfere with non-Mafia players. It would just increase the number of new threads in the subforum.
I could definitely see a limit of the number of games a player can participate in, but even that should probably be up to the individual on whether he can handle it or not. Robz apparently is a multi-tasking fool. Even then, I'm sure that his games suffer a little bit, but he seems to be handling it just fine. I suppose that's one benefit of playing by forum; you aren't under so much pressure to respond right away.
It would seem that if there are enough players interested in a new game, there's no reason to cap it at two simultaneous games. Since each "day" could last up to 2 weeks, it'd be unfortunate to have people waiting in the wings for so long.