User blog comment:B214/General Discussion/@comment-90.202.240.151-20161006174801/@comment-9595671-20161007010342

Actually Issei DID have a kid with Le Fay, but he was too young to come with them. The kids in EX pretty much said that the ones who did come were the only ones old and/or strong enough to do so safely.

Extending on that, aside from Elmenhilde if only because of their relationship isn't that far yet, all the girls he has kids with live in his house which now has Kunou as of Vol 22. We only know of half the kids cause that would spoil his future harem members.

As for Ophis, her page does say she's part of the harem while also being a mascot... but she's also genderless officially. So unless she somehow becomes female, Lilith's probably the closest thing we'll get to her and Issei's "child". Who knows, maybe Lilith's one of those 22+ kids?

While maybe some had multiple kids, I doubt he wants to show he favorites any of the girls more than the others and probably only has one with each unless one had twins. I mean the Vol spoilers mention him having Akeno as a wife so Rias is just his first one.

At the very least we do have candidates who can be his potential other wives. Kiyome, though Ishibumi's implied there's not much else from her, but maybe just in the main story. Tsubasa, who just needs to realize with the harem she can tell him and not be mauled by the others like she thinks.

Bennia... I'm not as sure cause I've only heard her be a fan of his show, but Le Fay is too so maybe I just haven't seen enough yet. As for Bina, if she's his queen officially in his peerage than I think she's in it. I think that's everyone so far, combined with the current harme members that's a total of... 12 official and 4 possible