McGrath, you're absolutely right as far as grassroots stuff is concerned. I was thinking more of "performing venues" situations (as my early message high up on the list mentioned), where if people don't show up and pay money, the event will not survive.
In the Calgary Singers' Circle, we've had as few as 3 people (a good session, by the way!) and as many as 39. For our contra dances, we have 50-80 dancers, usually. For both things, the publicity is solely a phone/email tree and word of mouth. Both events could survive with small numbers (except that for the dances we have to pay for the hall, also a dance with only a few dancers doesn't work well -- but we started with 8 people in a living room...).