How many "sub-levels" in one toy box?

I have been creating a toy box where I has become a dumping ground for my imagination and has quickly turned into a massive scaled world. I have achieved this with sub-levels but what concerns me is that I don't know what the limit is or if there even is one. I don't want to be too deep in creation and want to make another level only to find out that I reached the limit, so I would like to know ahead of time what the sub-level limit is so I'm not disappointed later.

You can have a total of 9 toyboxes connected to the main toybox. Which gives you a total of 10 combined.

I had a plan of using a toybox to corral all my works in progress, but then when I finish a particular world how can I copy/move it into it's own toybox?

@kc2dpt you can't. If you want a box to ever be separate it should be built separate.

What if I made a new toybox, put in a door, and "imported" from the first toybox? Can I import a sub-world from one box to another?