Looks like it counts number of revisions. Increments after every update.
Example: https://critiquebrainz.org/ws/1/review/?release_group=a3ea1232-72ac-4a71-8f5a-12c3c5622138&limit=1&sort=rating
Pull request submitted: https://bitbucket.org/metabrainz/critiquebrainz/pull-request/233.
Pull request submitted: https://bitbucket.org/metabrainz/critiquebrainz/pull-request/233.