Richie, I wouldn't keep more than 3 leos in a 20L tank simply because of the limited hide space available. I know there's that general rule of providing at least one hide per leo, but what happens if all 4 leos want to squish under the humid hide at the same time? It happens.
Having a larger humid hide would require a warm side covering nearly half the tank (in a 20L) if you want to provide warm dry hides as well. Or maybe I just haven't come up with a clever way to lay out the cage. :P
You may want to look into plastic caging rather than glass aquaria. They're lightweight and stackable, plus you get a different experience of keeping leos when you start keeping them in cages that open from the front. My skittish leos are eager to walk out onto my hand even though they never liked having me pick them up. Actually, they still don't like me picking them up but they'll climb onto my arm on their own accord. Their attention also stays focused on the one side that offers them a clear view of the outside world, so it's not unusual for me to wake up in the morning and see a friendly gecko face smiling at me.
