Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
If one has a legitimate excuse for missing fasts during Ramadan, then such a person has to make up the missed days after Ramadan.
The Quran clearly declares
“Yet if one among you is sick or is on a journey [such a person shall then fast] the same number of other days.” [2:185]
Unlike supererogatory fasting, making up obligatory fasting needs a specific intention. So, if you think you missed some days of Ramadan for some excuse when you were younger but you are not sure how many they were, then you need to fast, as makeup, a number of days that most likely cannot be less than the days you missed.
For example, if you think that the total days you might have missed could be around 15 days or a few more, but they can never be more than 20 days, then you need to fast 20 days.
However, if one does not have a legitimate reason for missing these fasts, then such a person has committed a grave sin which requires sincere repentance to Allah. According to the majority of scholars, still, this person has to make up the days he missed as detailed in the preceding case.
There is, however, another juristic opinion adopted by a number of scholars that observing many voluntary fasts compensates for missed days of Ramadan because these missed fasts can never be made up due to the lapse of their time limits.