There are 16 8 oz cups in one US gallon.
In one US gallon there are 128 fl ounces so since 8 times 16 equals 128 you would have 16 of those 8 ounce cups in one US gallon.
To figure out how many ounces are in a certain amount of gallons take the size of the cup in ounces and multiply that by the number of cups it takes to reach the amount of gallons.
So if you have 2 US gallons you would have 32 8 ounce cups in 2 gallons and then so on and so forth.