What is the ruling on a man marrying a woman if the woman's mother was suckled by the man's father's wife an unknown number of times, knowing that the man's mother never suckled the woman's mother?

If the girl's mother was nursed by your father's wife with your father's milk, she becomes your sister, and the girl becomes your sister's daughter. You then become her maternal uncle, and it is not permissible for you to marry her. This is unless the number of breastfeedings was less than five, or if you doubt the number, the default is that they did not reach the required number, and prohibition is only established with certainty. However, it is preferable and more cautious to avoid her, to steer clear of the disagreement of those who say that even one breastfeeding establishes prohibition.