As a general rule, one gallon of punch will provide about 13 5-ounce servings. However, depending upon the literal "size" of your guests, you will want to allow anywhere from two to four servings per person.
If you're also trying to decide on some specific punch recipes, there's a bunch of great recipes at www.birthday-party-ideas-101.com
If you're also trying to decide on some specific punch recipes, there's a bunch of great recipes at www.birthday-party-ideas-101.com