To what extent is Theravāda followed in Japan?
In Japan in 2003 the number of Buddhist temples were


30,000 Pure Land 
21,000 Zen 
15,000 Shingom 
14,000 Nichiren 
5,000 Tendai


The source for this is Introducing Buddhism by Peter Harvey (pp 409 2nd Edition)

There is a Japanese Theravada Association which claims thousands of members and the Dhammakaya movement has six centres in Japan. However this appears to be a more recent phenomena and the above figures appear to indicate that it is a small movement in the context of Buddhism as a whole in Japan.