Most party books and experts suggest a general rule for the number of guests to invite: your child's age plus one. It's a good guideline in theory, especially for younger kids, but can be difficult to follow in practice. You may want to bend this rule depending on the activities you plan. An even number works best for many games, especially team games, to ensure that no child is left out. As children get older, they can handle the excitement and stimulation of bigger groups. Rather than invite the whole class, you might invite just the girls or the boys, especially when your child is in elementary grades -- when boys and girls naturally separate and close friends tend to be the same gender. Younger children (7 and younger) and older kids (12 and older) often have friends of both genders, and coed parties can work fine for these age groups. Take the cue from your child; ask what feels right to him or her.
Whether the party is small or large, enlist help.The younger the group, the more assistance you'll need. For preschoolers, have at least one adult (or responsible teenager) for every six children; for school-age kids, assign one adult for every eight children. If you have an activity for which young children need extra supervision, such as an outdoor treasure hunt or a cooking project, add more grown-ups.
Start by asking parents to stay for the party, or recruit older children and their friends to help (offer nominal pay). Or consider hiring professional help; call day-care centers, community centers, friends' nannies, and baby-sitters you know. Tom Connor, a dad in Westport, Connecticut, who has eleven years of experience planning birthday parties for his son, recommends hiring young men in their teens and early 20s to supervise. "Most kids think guys that age are cool and will listen to them."