To equal 1 gallon it takes 7 and 1/2 500ml bottles to make a gallon.
There are 128 ounces of liquid in 1 gallon.
In 1 gallon there are 3785.41 ml per every gallon of liquid so since 5 x 7 equals 3,500 that means that means that you would need 7 and 1/2 500ml bottles of liquid to make 1 gallon.
To make 2 gallons of liquid from 500ml bottles of liquid you would need 15 of those 500ml bottles to equal 2 gallons.
Take the 500 and times it by 7 and 1/2 per gallon to get how many bottles in the 500 ml size that you need to equal more than one gallon.
Remember that 1 gallon always has 128 ounces.
If you have 8 ounce bottle or 8 ounce cups then you would need 16 of those 8 ounce cups or 8 ounce bottles to equal 1 gallon.