The amount of bottles of water you'll need to make a gallon of water will depend on the sizes of the water bottle that you have.
If you have water bottles that are 16.9 Oz in size you would need around 8 of those 16.9 Oz bottles of water to fill a gallon jug or bucket with the water.
There are 128 ounces of water in each gallon so you'll have to have that many ounces of water to make a gallon which is around 8 bottles of water that are 16.9 oz in size and you should have a bit of water leftover after filing a jug with the bottles of water.
So it could take a bit less than 8 bottles of water to fill a gallon.