There are several guidelines regarding the number of guests for parties.

Children's Parties

Some experts suggest inviting one guest for every year of the child's age. For example, if the child is four years old, invite four friends. Others say to invite the number you can manage and get help for larger groups. Some kids like to have the whole class over, while others prefer only one or two close friends, so you'll have to negotiate the numbers.
When you invite relatives and family members, the number of guests will depend on the size of your family. Just remember that children tend to be overstimulated during a celebration. Smaller groups can help keep the party atmosphere a little more manageable and pleasant.

Adult Parties

For adult parties, think about the amount of space available for your party in relation to the number of people you're inviting. If you're having a party outdoors, or in a large office, living room or party room facility, a lengthy guest list is fine. However, if your space is limited, keep the number of guests small to ensure that everyone will be comfortable.