One of the big things that come up when planning a birthday party is whether you should send invitations to the whole class. Schools have different policies and you should always follow their guidelines. Personally, I would be more inclined to not invite the whole class and just invite a few key friends. Do you really want 20 kids running around your house? A lot of people think, "the more the merrier" but I honestly think it just gets overwhelming for everyone. It can be especially overwhelming for the birthday child. Having a smaller more intimate party is more relaxing for everyone. An old rule to follow for children under 8 is their age plus 1. If your child is turning 7, plan on having eight kids attend. This is by no means a hard and fast rule and it definitely depends many factors. At a minimum you need to consider budget and location. Twelve is by far the most popular party size. Over the years as an Albany, NY magician this is the size I see more often than not.
Being prepared and organized is your best bet no matter how many children you invite to your child's birthday party.