How many RTK'ers are actually in Japan?
A bunch, actually. According to Google Analytics there are 82 unique visitors from Tokyo alone. That's the big orange dot on the asia map below. Note that does not mean 82 people who actively use the website, neither does it mean 82 people who have registered. But I'm willing to bet at least 20 of them are. I've noticed at least one school of Japanese learning in Tokyo which linked to RevTk in their blog.
It surprised me very early on when I saw many hits from Japan, but then it's really obvious. A lot of westerners living in Japan, and particularly students going abroad are giving RTK a go.
Here's some cool maps from Google Analytics '"Geo Map Overlay" :
Europe :
Big dots : Sweden/Goteborg (39), Belgium Oud-Heverlee (37) (probably around Brussels), UK Arnold (38), UK Southend-on-sea (27), Italy Rome (23), ..

Us :
Big dots : "oceanside" (what's that?) (37), Caldwell (41), Montreal (33),

Asia :
Big dots: Tokyo (a whopping 82!), Kodaira (42), Wako (36), Nishinomiya (26), Oita (21), Yoka (15), .. it's difficult to read the dots because they are all stacked on top of each other 

Full map :
This one is cropped, but there are no orange dots outside, except for Honolulu, which is in the middle of the ocean outside on the left (2 visitors
).

Most of the small dots in other places are ~5-15 people.
I think those are the locations of the servers or routers, they are not the exact places where people connect from but pretty close.
Enjoy 