1 gallon of milk contains 128 ounces.
So you would get 128 ounces of milk out of a gallon of milk and the amount of glasses of milk in the gallon of milk will depend on the size of the glasses you're pouring the milk in.
But let's assume you are pouring the milk into 8 ounce cups or 8 oz drinking glasses.
If you have 8 ounce drinking glasses you're pouring the milk into then you would have 16 glasses of milk out of the 1 US gallon of milk.
So for 8 ounce glasses you would be able to get 16 glasses of milk out of the 1 gallon of milk.
The larger glasses the less servings of milk you'd be able to get out of it.