I was told, bsheym Rav Yaakov Kamenetzky zatzal, that the source for lighting one candle per child is that it used to be that after childbirth every mother was in the hospital for a minmiumum 10 days.
A similar idea is expressed by the ??”? ???? ?????, as quoted by the ????? ??? ??????. This is, in fact brought as proof that even missing ????? obligates an extra candle. The ???? ????? doesn’t hold like this, though.
There’s a question on this approach; why would she add another candle for missing because of childbirth since her husband lit, and they were both ?????
I would suggest that a ???? might have developed for some, that a woman add an extra candle even if it was an ????, and even if she was otherwise ????.
This would explain why some light extra even if not at fault for missing.
I’ve seen another explanation for the ???? of adding a candle for each child. The ???? in ??? says ????? ??? ????? ??? ???? ?????? ????? (that the ???? of ???? helps one have children who become or marry?????? ?????), so as a ????, each time a child is born, a candle is added as a ????.
According to either of these explanations, a woman would not light according to the number of her husband’s children.