The freezer is too narrow to fit 2 deep, so you need to stagger them. I'm sure I could squeeze a 4th, but it would be super tight.
My plan is have 3 and the co2 tank on the floor, then put bottles on the hump.
Of course you also could put a 2.5 gallon on the hump, or a high collar and get a 5 g there. I'm not going to put a high collar because I'm putting an irish coffin on the top.