Let's play with numbers!
I think I remember Karyuu saying at one point that there would potentially be well over 200 different maps in the game. I think right now we have something like 11. If we're at half of the upper tier, then that means that there's another 9 or 10 maps which will be added. If we have another 7 levels, and each one is smaller than the one before, and the biggest one has 20 levels, and each level has two fewer maps than the one above, then that's 18, 16, 14, 12, 10, 8, and 6 maps. That's over 100 maps for the stalactite, and that's probably at the very smallest. Then there will be the tunnels leading outside of the stalactite which may even lead to the surface, and I can imagine that could involved over a hundred maps as well. And we've also been told that the death realm may be just as big as the living world. So maybe there would be over 300 maps at the very least. And if we have around 11 right now, then that's around 3-4% of the total world.
Of course, none of that means anything. I'm just having some fun with it. :-D