The number of water bottles in a gallon depends on the size of the water bottle. A gallon is 128 fluid ounces; one must divide 128 by the number of ounces in the water bottle to get the answer.
A gallon of water by definition is 128 fluid ounces in the United States. Water bottles can vary in size, but the total number of bottles making up 128 ounces can fit in a gallon. For example, if the water bottle is 16 ounces in size, then 8 of these make up a gallon. If the water bottle is 32 ounces, then only 4 of these make up a gallon. A 1 liter bottle of water is approximately 33.8 ounces, so approximately 3.8 of these bottles make up a gallon.