What is incumbent upon one who did not make up for the days of breaking the fast in Ramadan and does not remember their number or the date of their beginning, and is it obligatory to make up for missed prayers, or is sincere repentance sufficient?

All praise is due to Allah for guiding you to prayer after sometimes abandoning it. You must repent to Allah for your previous negligence in prayer and strive to maintain it in the future. Scholars differ regarding the ruling on one who abandons prayer; some consider it an act of disbelief. The majority of scholars hold that you must make up all missed prayers. If you know their number, then make them up. Otherwise, continue making them up until you are absolved. Make up prayers daily according to your capacity, in a way that does not harm your body or your livelihood. You must also make up the days you missed in Ramadan (definition). If you do not know the exact number, continue making them up until you are absolved. You are not required to pay expiation for delaying the make-up if the delay was due to ignorance or forgetfulness. And Allah knows best.