User blog comment:B214/Volume 22 spoilers/@comment-27083298-20160716005354/@comment-1359322-20160716024458

The last time Ishibumi said a volume will have more illustrations is Volume 19 and that only has 5 new illustrations and 3 recycled illustrations. This time around, Ishibumi didn't announced anything regarding the illustration, as a matter of fact, after that Volume 19 "special", Volume 20 & 21 reverts back to 4 illustrations. So 4 illustrations is currently the norm in DxD. But we also got cases like Volume 18 where there's only 2 illustrations, so i can't be putting any hopes on there being more than 4 illustrations.