As regards the question, it is noted that delaying making up for missed days of obligatory fast without an excuse is a sin. Thus, delaying them without an excuse, requires repentance to Allah, seek his forgiveness, and never repeating such again and to fast the missed days.

If a Muslim cannot remember the exact number of days, then fast what he thinks is most likely. If he think sit most likely that the number is thirty days, for example, then he should to fast these days.