2 Answers

As you can see by the camelback link below, there are many different water bottle sizes. A gallon contains 4.55 litres. If you had one of the 0.6 L water bottles you would need 7.58 of them to make a gallon.
There are 128 ounces in a gallon. If you had one of the 21 oz water bottles you would need 6.095 bottles to make a gallon.
The camelback packs also come in several sizes, but generally much larger than their bottles, so you would need fewer of them to hold a gallon.

it would depend on which water bottle it is, they all have different volumes