User blog comment:B214/Kuoh Academy students finding out that Issei stays with the ORC girls and is dating Rias./@comment-27083298-20160822122252/@comment-24098614-20160822123625

This comment already shows that Volume 22 is the end of the series because if his friends have found out it means no more Dxd, and I don't foresee this series continuing after Volume 22 everything has been accomplished.