Calculating your occupancy load is a pretty simple task, but it depends on what the event looks like. If people will be sitting through presentations, and the room is heavy with furniture, you can figure 15 square feet per person. If you are hosting more of a reception or a conference, and the room is relatively free from furniture, you can go with 7 square feet per person, since people will probably be on their feet most of the time. The design of the room will give you a good idea of how people will utilize it.