Try planning a wedding, conference, bus trip, or dinner party where an exact head count helps, if it's not completely mandatory. You could be an ass and say "s

Try planning a wedding, conference, bus trip, or dinner party where an exact head count helps, if it's not completely mandatory. You could be an ass and say "sorry, no RSVP, no entry", but most people will try to show a modicum of etiquette more than the guests and accommodate the fools somehow.