If you’ve already signed the contract there’s not much you can do but either add to your guest list or pay the per person rate for the number of empty chairs.
It’s very important that you review contracts with all of your vendors and make sure you really understand what’s in them. Don’t be afraid to have your parents, his or a friend with experience in contracts review them too. I’m not suggesting that you hire a lawyer.
In the case that you received a larger number of regrets than you expected, and that’s the reason you won’t hit your minimum, don’t be afraid to invite additional guests. You can either be honest and explain that the space had a very limited number of seats so you were very limited with the number of people on your guest list or fib a bit and tell them their invitation came back in the mail and you want to “resend” it to them.