Mafia isn't any fun if half of the roster is severely inactive.
Failing to confirm night actions, or to display some level of message activity (player dms, the mafia server, and the main mafia chat will be taken into consideration) may be punished with a modkill, possibly along with a ban from any future games that I run.
There won't be some hard set limit, it's up to GM discretion. If you don't feel like you can keep activity up, don't play!