User blog comment:Master Sima Yi/Assassinews 10-05'12 — World Gameplay Premiere/@comment-184.146.156.246-20120512042337/@comment-1153722-20120513082150

Clay Kaczmarek is Ezio's descendant, but could view significantly less of Ezio's memories than Desmond did. This means that Clay was the descendant of a child he had years before. Maybe that Amelia girl got pregnant, or one of the courtesans after Marco Barbarigo's assassination. But we don't know if Ezio had "many" illegitimate children, it might just have been the one.