From my experience I buy gallon jugs of water to pour into my 12 cup coffee maker to make better coffee because I don't like the city tap water in my coffee.
So when I use the gallon jugs of water in my 12 cup coffee maker I can make 2 pots of coffee with 1 gallon of the water.
So in my experience there are 24 cups of coffee in one gallon so if you have one gallon of coffee you have at least 24 cups of coffee.
I can make two 12 cup pots of coffee with 1 gallon of water because it only takes 1/2 gallon to fill my coffee pot with 12 cups of water.