Praise be to Allah, The Lord of the Worlds.
Allah has granted man the power of divorce, and he is to be checked with in this regard. Thus, if he is unsure whether he had divorced his wife or not then she isn`t considered divorced. This is because the marriage contract is certainty and, in principle, it must stay as such based on the famous jurisprudential maxim "Certainty isn`t removed by doubt."
According to Shafie and Hanafie jurists, if the husband was certain that he had divorced one of his wives, but forgot which, then he must stop making love to them until he remembers. Kindly check {Tohfat Al-Mohtajj, 8/70} and {Bada`i` As-Sana`i`,3/229}.
Maliki jurists hold the view that if the husband forgot which of his wives he had divorced then all of them are considered divorced. Kindly check {Minah Al-Jaleel, 4/144}.
One the other hand, Hanbali jurists are of the view that he should toss a coin. Kindly check {Khashaf Al-Khina`, 1/50}. However, Ibn Qhoddamah, a Maliki jurist, holds the opinion that the coin should be tossed to decided which inherits him, in case he had died, and not which he had divorced. {Al-Moghni, 7/497-498}.
In conclusion, if the husband had divorced one of his wives then forgot her, he should cease making love to all of them until he remembers her. Moreover, Islamic authorities are in charge of issuing rulings on such cases. And Allah The Almighty knows best.