In 1 gallon there are 16 eight ounce size cups.
In 1 gallon there are 128 ounces of liquid so since it takes 8 x 16 to equal 128 it would mean that you would have 16 of those 8 ounce cups in one gallon.
The average size cups are 8 ounces but if you have a smaller cup then it would take more than 16 cups to make a gallon and if you have larger than 8 ounce cups then it would take less cups to make a gallon.
But the short and simple answer to the question on how many cups are in a gallon is 16 cups.
16 cups are in a gallon.