D.A.
I have taught children from 1 year to age 15, as well as having two of my own. Early on, I discovered, and found agreement with other parents and professionals, that it will be a much more pleasant experience for any birthday party to have a limited number of children: generally, as many children as the child's age. So, if he's going to be 3, have no more than 3 guests. If he's going to be 9, have no more than 9 guests. If the child happens to be shy or has special needs, this may be too many. But it's rare that it works well to have more children than that basic guideline unless the party is managed by a professional educator who knows the interests of the children and can plan accordingly, as teachers do for classroom parties. You'll have much more fun if you limit the number of guests.