What is the ruling on someone who owes old Ramadan fasts and does not know their number, so how should they make up for this debt?

Fasting days missed by a person before puberty do not have to be made up. The same applies to days where there is doubt whether the fasting was broken before or after puberty. However, if it is certain that fasting was broken after puberty, those days must be made up. If there is doubt about the number of days, one must make up the number of days that ensures certainty or strong probability of fulfilling the obligation. There is no expiation (fidyah) required if the person was unaware of the prohibition against delaying the make-up fast.