4
But is that implicit in the flag
Not directly, but a Not An Answer flag puts it in the Low Quality Posts review queue, and users there will be invited to leave a comment. They can even choose from standard ones ('canned comments'):
That said, I often leave a comment as well. The user script AutoReviewComments - Pro-forma comments for SE helps me because it'...
Only top voted, non community-wiki answers of a minimum length are eligible