1 gallon jugs contain around 128 ounces of liquid so you would have 128 ounces in 1 gallon.
If you serve a gallon of liquid such as tea, milk, lemonade etc in 8 ounce glasses or cups then you would get 16 glasses, cups or servings out of the 1 gallon jug.
So you can expect to serve at least 16 people with 1 gallon of some sort of beverage.
But you should always have around 128 ounces in 1 gallon jugs.