We just had our daughter's 5th birthday party so I get where you are coming from. We had her party at a pottery studio and we had to pay for a set amount of kids as well. We invited all of the kids in her preschool class (invites to 22 kids) and we did the invites 3 weeks out from the party so people would have time to respond if they were on vacation, etc. I got 2 RSVP's the first week and didn't get any more until a few days before her party. One mom called to say they were coming and then they didn't show. No phone call or email to let us know either --- RUDE. All in all, out of 22 invitations being sent out, we had 9 RSVP that they were coming (only 8 showed up). Out of the 13 other kids that we invited, only the mother of one child called to say that they were going to be out of town and couldn't come. The parents of 12 kids didn't respond in any way at all.
I ALWAYS call or email to RSVP or give our regrets that we can't make it. My daughter got sick last month and was sent home from preschool the day before we were supposed to go to one of her friends birthday parties. At least I had the decency to pick up the phone and call to let them know that we couldn't make it, unlike our no show.
I have two more pet peeves:
1. A majority of the parents don't write thank you notes for the birthday gifts you give their children. I think that is so rude.
2. Invites sent out with very little notice. We just got an invite yesterday, Tuesday, for a birthday party this Saturday. We had other plans, but this is for a little girl my daughter is really close to, so now we're going to have to change our plans and scramble to get a birthday gift. I think it's nice to give at least 2 weeks' notice (maybe 3 weeks in the summer when people are in and out on vacation).