To add to that, the Touhou Loseless Music Collection torrent on the cat website is currently at 3.3 TiB, and features more than 1900 different circles (bands I guess?). I think if you count someone playing a song on a guitar as a cover he's probably not the most covered artist of all time, but if you measure it on availability of hobbyist covers, he might very well be.
Another data point: The Reitaisai[1], a convention focused solely on Touhou Project, where hundreds if not thousands of circles (groups) sell fan-made content, including music.