User blog comment:B214/Volume 22 spoilers/@comment-90.202.240.151-20161031050614/@comment-30048084-20161031165443

I think Ishibumi made the DxD part primarily to refer to Great Red and also what Sky-dragon-twilight said about the large amount of dragons in the series afterwards I think that Ishibumi just kind of ended up creating more DxD's for fun and connecting it to the name of the series.

If we do a round up of dragons

1 True Dragon + 2 Dragon Gods + 1 Humanoid Dragon [True Dragon Demi-God] + 2 Heavenly Dragons + 5 Dragon Kings + 3 (Heavenly) dragon class evil dragon + 4 other evil dragons = 18 named dragons and an unknown amount of unknown dragons