User blog comment:B214/General Discussion/@comment-9595671-20160820093528/@comment-9595671-20160823045333

That is a possibility, it's just unlikely since each mother in EX has only had a single child with Issei, and Issei's job currently makes it hard for him to have any time to have any more.

That would be another thing a second part of EX could help clear up for us. Answer if any of the mothers had multiple children. For one, we never saw if he had a child with Kunou, and since Kunou's moved into the Hyodou household I think it's cemented she's part of the harem.