What is the ruling of Islamic law on a man who divorced his wife and is unsure about the number of pronouncements, whether it was two or three?

If a husband doubts whether a divorce occurred or regarding its number, he must base his decision on certainty. If he doubts whether he divorced or not, the default is that no divorce occurred, because the marriage is certain and the divorce is doubtful, and "certainty is not removed by doubt." If he doubts whether he pronounced one divorce or two, he should consider it one, as that is what is certain.