If you mean total attendance is 300 people, I think you can get away with less ribs than a normal vending situation. Incidentally, the math doesn't pencil, three BBQ vendors for 300 people is pretty poor odds of making money, even less if there is other food.
Do you mean that you have to prepare 300 full portions, or assume that you are fighting for a share of 300 total attendees? Unless you have a large cooker, cooking for 2 bones for 300 people, you will need 50 racks ready for serving. That is a lot of space and coordination.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less