What is the expiation for not fulfilling many previous vows when their number is unknown?

It is obligatory for a Muslim to fulfill his vow if he vowed an act of obedience to Allah the Almighty, as the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah the Almighty, let him obey Him." If a Muslim forgets what he vowed, he must investigate and strive until it becomes clear to him. If he predominantly thinks it was, for example, prayer or fasting, he should perform what he presumed. However, if nothing becomes clear to him, then the prevailing opinion among scholars is that he must offer an expiation for breaking an oath (kaffarat yamin (definition)).