These days I always ask how many children will be at the party for the magic show. If I am doing balloon animals I need to make sure I have a balloon for the last child at the last party of the day.
It also helps me know what kind of crowd to expect. Will it be a packed house or just a handful of kids. I like a large group of kids. Much more energy comes at me and I bounce it back. It changes the show. I suppose if my show was one where the kids sit and watch it like a TV show it would not matter. But my shows are very interactive with the kids. The birthday child gets to come up and help make the magic happen, many of the other kids get to come up also and all are encouraged to yell out the magic words and make magical gestures. They participate. This is what makes it worth going through the expense and trouble of having live entertainment!
Years ago I didn’t bother asking how many kids would be coming to the party. It didn’t matter to me. As long as I had room for my table to set up I could handle any size group. It was true but now I like to be forewarned. Why? Because of the Party of Five.
I was invited to perform at a celebrities house. This will be fun I thought, a big lavish party with lots of kids. But I was wrong. I performed my magic show for the birthday girl, her two brothers and both parents. The energy level was totally different. Being loud and boisterous is not really appropriate which you have to be for a large crowd so they can see and hear you in the back. 
I actually did some of the magic I do when walking around doing magic for small groups of people. Card tricks, rope and coin tricks. Things in hands. I taught the older brothers how to do some tricks that I teach in my magic classes. A fun magic show but different.
So now I ask how many kids will be coming even if the parents can only give me a ball park figure. I can do a birthday party show for five, fifty five or more but it becomes a different show. Forewarned is forearmed!