There are eight 16 oz cups in a gallon.
For every gallon there are 128 ounces so since 16 x 8 equals 128 you would have 8 of those 16 ounce cups in a gallon.
So if you have a gallon of Juice and you serve the juice in 16 ounce cups then you would be able to serve 8 people with 16 ounce cups of juice.
If you need to serve 16 people with that 1 gallon of juice then you would need to use 8 ounce cups because you could then serve 16 people with 8 ounce cups of juice out of the one gallon of juice.
If you have 2 gallons of the juice then you could serve 16 people with 16 ounce cups or serve 32 people with 8 ounce cups out of 2 gallons.
There are always 128 ounces in 1 gallon and in 2 gallons there are 256 ounces.