First - Get a contract for a guaranteed number to save any heartache later, and agree what will happen when more show up. If it's 100, put out only 100 plates. To make sure folks don't double up, do a served buffet with a helper or two to move everyone thru the line. Include a dinner roll to take up plate space.
Cooked onsite, 100 folks for this menu, I would go $16 a head.
Just me, the Wife, and the Dogs