User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-177.142.210.225-20151120133208/@comment-1359322-20151123235016

Because Ishibumi already said he'll make another volume for Rias, Asia, Akeno, Koneko, Le Fay, Kuroka & Kunou. That's 7 volumes in total, add Volume 21 and you'd get at least 28 volumes.