User blog comment:B214/Volume 22 spoilers/@comment-213.120.234.106-20160720102919/@comment-9595671-20160721041831

Atleast the ones we know of, I don't know how accurate the info I've seen is, but I recall seeing one source that says Issei has atleast twenty kids.

EX only showed us half of them and I know there's atleast 3 others at the top of my head who like Issei that he also could have had children with.