To serve 100 people or 100 guests with punch you should have at least 7 gallons of punch to serve those 100 guests.
However you really should have at least 14 to 15 gallons of punch for 100 guests just because they may drink more punch than you expect.
There are 128 ounces in every gallon of punch so lets say you were serving each person 8 ounce glasses of the punch.
If you were serving each person one 8 ounce glass of punch then you could serve 16 people with one gallon of that punch.
So if you have 100 people to serve and you're serving the people 1 eight ounce glass of punch each then you would need 7 gallons of punch minimum and you'd have a bit of punch leftover if you only served one glass of punch each.
However I would get at least 14 to 15 gallons of punch to serve 100 people just to make sure you have enough punch to go around.
It's better to have too much punch than not enough punch and you could take the rest home with you to drink later.