According to Serious Eats, a gallon of ice cream can feed approximately 30 people. Typical ice cream scoops dish out about 3 ounces per scoop, though different sized scoops exist and can cause variations in the number of people that can be fed from a single gallon.
Scooping ice cream is not an exact science. The amount of ice cream in a scoop can change depending on who is doing the scooping and how deep the scoop digs into the container. A kitchen scale can be used to make sure that every person gets an identical amount of ice cream.