1 gallon of lemonade has 128 ounces of lemonade in the container.
So if you had 8 ounce glasses of lemonade you would be able to serve 16 people from 1 gallon of lemonade if you served them 1 glass of lemonade.
If you have 16 ounce glasses of lemonade then you would be able to serve 8 people one glass of lemonade per each 1 gallon of lemonade.
It all depends on the size of the glasses you're serving the lemonade in as to how many people 1 gallon of lemonade will serve.
But you should be able to serve between 8 and 16 people with 1 gallon of lemonade depending on the size of the cup you're serving the lemonade in.