even if all you put in the tank is 2 geckos, as long as they get along, they will LOVE the extra room. Personally, if I could afford to turn one of my rooms into a terrarium, I would.
If it's 2' tall then you may have room to add a shelf about a foot up like a loft thereby increasing your surface area and allowing juveniles to climb to the second tier and avoid the larger dominant adults. Then you could go like 8 to 10 depending on size. The most important things to consider are an adequate number of hides and a properly visually sexed group. Males should never be housed together and females need the option to hide in a group or alone. A good rule of thumb is one less hide than you have geckos in the cage. I'll be posting some photos soon of my shelves and how they serve as added inhabitable surface on my site. They will like the extra space and be more active for it. A large water dish will give them the option to soak or drink and help to maintain humidity in the cage. Sounds like it could be a nice setup. Good Luck