User blog comment:B214/Volume 21 info & spoilers/@comment-128.153.198.58-20160315012244/@comment-26821288-20160315150834

Volume 21 isn't going to be the last Volume, not at all. Ishibumi already talked a little bit long ago about Volume 22 already, and as you can see in "Future", Ishibumi also said that Volume 22 will begin the Fifth (and last) Story Arc, so Volume 21 isn't going to end High School DxD, and also Volume 22 won't end High School DxD.

Long story short, this Light Novel series, High School DxD, has (probably) still a long way to go with us (and I hope so, too) and isn't going to end soon (or at least, so I hope).