The figure of 400 is incorrect - see the final group of posts in this thread:
I’m not sure how anyone can calculate even an approximate number, however. As has been pointed out there are many Roon users who don’t post in these forums or contact Roon support.
As I understand it there have been two principal sets of issues: dealing with corrupt databases (the subject of the current complaint) and missing albums. For many, such as myself, the latter was fixed by the build 882 update.