The amount of servings in 1 gallon of lemonade will depend on the size of the cups you're serving the lemonade in.
But most people serve lemonade in 8 oz cups so if you serve the lemonade to people in 8 oz cups you can get at least 24 of those 8 oz cups out of 1 gallon of lemonade.
So you have at least 24 servings in 1 gallon of lemonade so if you're serving 24 people you need at least 1 gallon of lemonade and for 100 people you'd need a bit over 4 gallons of lemonade.
But to be safe it would be best to have 5 gallons of lemonade if you're serving 100 people.