I can easily fit 3 dogs in the back of my TJ which has no back seat and they are more than happy back there. I checked out a new JK 4 door and was amazed at how much room it has in the cargo area so with the back seat down I would say 5 or 6 dogs will fit. Sure they will bump into eachother but once they get a good spot they will stay put and be happy. I've put 1 dog in my TJ when there was barely enough room for him with all the crap I had in the back and he was as happy as could be.
I'll have to post a picture I took this summer of the dog (golden lab) while doing 80mph down the highway. His lips were flappin in the wind and his eyes were rolled back and we took a shot of him by aiming the camera at the side view mirror so you got the "Jeep" on the mirror shot in the picture. Classic! Only in a Jeep!