Look up "crowd manager" requirements in your jurisdiction. In mine, one is required for every 250 occupants in clubs and bars and such - A2 and A3 assemblies. Not required for fixed seating style venues. Training is required. This is sort of different from how many crowd managers and ushers
would be a good idea, but at least a starting
point for bare minimum. Beyond that, enough to
cover each aisle and have eyes on the whole
house and
lobby and perhaps parking lot.