I would say to figure about 1 large tomato per 3 to 4 burgers. It depends on the size of the tomatoes. For the onions, it would depend on how many slices per burger and how thick. Most people use 1 or 2 slices of onion and 1 slice of tomato. Also, I would slice both onions and tomatoes thin for this many people. That's a bunch!!!!! Good luck!!!