My gaming group consists of roughly 10 adults, each able to commit to any given gaming session only when the vagaries of their schedule allows. We run two campaigns at a time, taking into account the availability of two GMs and whichever player has a character who is a ‘hinge’ to the upcoming session. Sometimes, we cannot get a session of either campaign together but still have 3 or 4 players. In the last year or so, I’ve finally acknowledged that the only effective way to pull out some quick gaming when such an attendance squeeze arises is to pick a backup system and pull out a module.