A:Answer Without completely taking all the food out of my own fridge and moving shelves around, my best guess is 15. You can get 3 gallons in the door, and you can go 3 wide and 2 deep on the bottom and middle shelf for 12 more. I can’t tell for sure but it looks like in this configuration you might not be able to get the top shelf in and fit the milk on the second shelf. So you can fit 15 gallons of milk and that’s probably all you’ll get in the fridge. Or, you could fit 9 gallons of milk, and have the remaining two shelves for the rest of your food/beverages.
If you wanted to be more creative and use some large flexible camping bladders, you could theoretically actually fit closer to 50 gallons of milk. Good luck!