Hopefully you figured it out, but I think 2 is the max number in/on the pen, although you should be able to continue feeding them in your inventory (and can equip them as pets). I think the pen is just there as a tutorial effectively and not really intended to be used except for the first two. I don't remember if you can feed the baby dragons on the pen more but you can remove them (and there are other locations you can place them), although I had an issue at first finding the right spot to open up the menu. I'm not certain but don't think there is a difference between the demo and full game other than that you can't leave the first world in the demo.