The amount of punch required to serve 100 people will depend on a number of factors. If punch is the only beverage served, is non-alcoholic and the party is expected to last four hours, 31 gallons of punch should suffice. If the punch is alcoholic, 29 gallons are needed.
It is estimated that each person will have 5 cups of a non-alcoholic beverage in four hours. You can calculate by multiplying the number of guests by the number of drinks by the number of ounces in a drink and dividing by 128 to convert ounces to gallons. In this case, 100 people times 5 drinks each times 8 ounces (standard cup size) = 4000 ounces / 128 = 31.25 gallons.