Apparently there are more than 1 million releases where none of the cover art pieces have been marked as front.

• https://musicbrainz.org/statistics/timeline/count.release.coverart.none
• https://community.metabrainz.org/t/544812#a-nameamazon-cover-artaamazon-cover-art-20

Currently there is only a report for releases where none of the cover art pieces have any type set but the results not even close to that,