Some events are space-limited, usually due to venue restrictions, regulations, or venue size. If a location gives us permission to bring 40 people, we limit the event to 40 photographers. There is no margin for no-shows. Only jerks no-show for an event that has people on a waiting list.
If a location is smaller, we limit the size so that people who can attend will be able to shoot without getting other photographers in their shots, stepping on each other's toes, or tripping over each other's tripods.