Make the reservations for the maximum number of people who might be showing up. Be sure to include infants and even those who might just sit at the table and not eat.
Showing up at the restaurant with extra people not on the reservation could be a problem. It will depend on the restaurant and how busy it is when you arrive, as to whether the castmembers can accommodate or not. You might be fine and all be seated, you might have to wait longer or the restaurant might seat only the number of people on your reservation so you'll have to decide among yourselves who is going to eat elsewhere. If you all decide to leave you might be charged the no show penalty. As much as possible the restaurant will try to seat everyone, but at the more popular venues it might not be possible because they are already fully booked and the kitchen is working at capacity.