For every 1 gallon of water or other liquid there are 128 ounces.
So if you have 1 gallon of water you have 128 ounces of water.
That would mean that it would take 16 cups that are 8 ounces in size to make 1 gallon of water since 16 times 8 is 128.
So if you have 1 gallon of water you can get sixteen 8 ounce cups of water out of 1 gallon of water.