Reply to Birthday party RSVPs

I agree with the people in support of RSVPing. I always do party favors, and I would hate for a kid to be left out because there are not enough, but I also don't want to buy 20 if only 5 kids are attending. I also like it when parents *ASK* is siblings can also attend and not assume that they can. I don't mind- as long as I know in advance- but again- I need to know numbers to make sure there are enough cupcakes and party favors for all the kids.