User blog comment:B214/Volume 22 spoilers/@comment-207.96.205.3-20160802182420/@comment-73.160.58.215-20160802184717

Information from the end of Volume 21 states the intentions to switch from the "Four Great Satans system" to a "Seven Great Satans system". Futhermore, on Ishibumi's blog there is an information page that was updated to include the seven "new" Satans: Lucifer, Beelzebub, Leviathan, Asmodeus, Belial, Belphegor and Mamon; with Ajuka remaining as the Satan Beelzebub. This also matches the information brought back from an alternate future by Issei's kids in the EX side stories.