I'm not sure if adding another dragon dimlet will increase (have not tried), I've only ever seen a max of 6 dragons at once but usually only have a max of 4 at once.
If I spend alot of time in the dimension it seems to go up and down from 1 spawning up to 4 over the day/night period.
If you hang around the 0,0 location of the dimension they will all go there trying to do that rest thing they do in the end over the end portal, only 1 can be at 0,0 at a time, so the rest will just hover next to that location waiting to get in. - good place to kill them.
edit: I have also noticed that the dragons at the edge of my view (rendering 16 chunks) will stop moving and be frozen - I think the chunks at the edge of view are no longer loaded as when I move closer the dragons start moving again.