Last year for DD's 5th it was first term of prep so I invited the whole class as DD hadn't made any real friends yep. We had it at a gymnastics school and the price was per head with a minimum number. I asked for RSVPs a week before the party and hardly got any. So I invited some more kids to get to the minimum number and yes you guessed it the day before the party at least 15 parents sent me texts/phoned me to say "oh sorry I'm a bit late to RSVP but x is coming to the party". I was livid and still getting "yes" RSVPs a couple of hours before the party! At least one kid turned up who hadn't replied at all. Grr. Everyone thinks oh it won't matter if there's just one more... But when the majority do it then yes it does matter!
There ended up being 25 kids, it cost me the earth but DD had fun (I think).