Did a manual count yesterday because I was too tired to do anything that required actual awareness, and the number of screens was 557 (hand-built, accessible, non-repeating). +1 now since I just made another.
Note that that number does include a few screens containing nothing but the upper part of some background feature like mountains, trees, roofs. And a few screens that have very little built stuff on them, just the corner or the top of some climbable feature. But mostly it's properly "walkable" screens.
A number that may be even more interesting than the screencount: FW pt. 2 currently has around 35 distinct areas. I say "around" because "distinct" is actually a tricky concept here - lots of internal variation, lots of transitional areas, etc. Kinda hard to draw boundaries around areas in this thing.
There's three or four major gaps to fill in the lower part of the world (4-5 areas, I'd say), plus the top layer of the world (5-8 areas, probably).
I'm currently mostly filling in small gaps and am looking forward to getting started on actual new areas soon(ish).
As you can see I've grown rather careful with screenshots, haha. I'm kinda glad most of the links to the screenshots in this thread are broken nowadays. Would ppl like more screenshots here again? I could oblige, but I'm not sure if it's a good idea.