Imagine a world in which being the seventh son of a seventh son (or whatever combination of number and gender) reliably made you magical. The result wouldn’t be as awful as a “magic gene,” a la Star Wars, but it would still do some pretty weird things to human evolution, not to mention society.
I imagine there would be a pretty high selective advantage for fecundity, and kin selection would play a role too, with the non-magical siblings of the magic Xth child being granted advantage due to their magical largess. Outside kinship circles, though, competition would be intense, with everyone scrambling for the resources necessary to produce the proper number of kids. And what do you do with all those excess kids?
I’m thinking small constantly feuding agrarians, periodically united/oppressed under ferocious “witch-kings.” Fun!