What is the expiation for the oaths I broke in the past, the number of which I do not know?

If the oaths are about a single matter, then only one expiation is required, based on the statement of Ibn Umar, may Allah be pleased with them both: "If you swear an oath multiple times, then it is one expiation." However, if the oaths pertain to different matters, then an expiation is required for each type, if the number of broken oaths is known. Otherwise, caution must be exercised.