I just purchased a 36x18x12 terrarium for my 2 fire bellies (I am getting more eventually, that's why the size). I don't know what size in gallons this terrarium size is equivalent to, but I was planning on a total of 6 fire bellies.

How many FB's can comfortably be housed in a terrarium of this size?

Would a large heat pad be sufficient for heat? I'm using a small one on a small tank them now and I'm getting adequate temperature.
(I want to be able to keep them on a shelf needing just 13" H)

I was looking at having an aquatic section about 15" wide. With some lily pads and maybe real plants but problem would be getting lighting there is I don't have the height requirements on the shelf.