There are 128 ounces in every gallon of lemonade so lets say you were serving each person 8 ounce glasses of lemonade.
If you were serving each person one 8 ounce glass of lemonade then you could serve 16 people with one gallon of that lemonade.
So if you have 100 people to serve and you're serving the people 1 glass of lemonade each then you would need 7 gallons of lemonade minimum and you'd have a bit of lemonade leftover if you only served one glass of lemonade each.
However I would get at least 14 gallons of lemonade to serve 100 people just to make sure you have enough lemonade to go around.
It's better to have too much lemonade than not enough lemonade and you could take the rest home with you to drink later.