As to make it even more understandable, the game works as follows:
You navigate to a screen that you want to make content in. This screen is adressed by its x and y value. The default screen you get when you start a new room is located at x1000y1000. This is already quite something. Its not x0y0. So you can go 1000 screens up and left from that location if you wanted to. This alone, 1000x1000 are 1.000.000 screens. Now obviously, you can also go all the way to x9999y9999, but as Nifflas pointed out, you can go higher too.
So lets assume that x9999y9999 would be the highest screen number, and x0y0 would be the lowest. you would have 10.000 screens on both axis, giving 100.000.000 screens.
The game was made in such way that you shouldn't be facing any limitations in what you can do as in extending your work. The only limitations that I can think of are the fact that you can only have 2 tilesets on one screen, but that also isn't really that much limiting either. I usually only need one tileset in the full extend, and some tiles from a second tileset.