Is it obligatory to make up for missed fasting days from previous years, given the uncertainty of their number or whether they were made up?

All praise is due to Allah for your return to Islam. You must seek Allah's forgiveness and make sincere repentance for the apostasy and negligence that occurred. Learning and practicing a religion other than Islam is considered clear apostasy, and no deed from its perpetrator is accepted until they repent and return to Islam. Allah Almighty says: (And whoever desires a religion other than Islam, it will never be accepted from him, and in the Hereafter, he will be among the losers) [Quran 3:85].

If you have repented and returned to Islam, you are not required to make up for the prayers or fasts you missed during the period of apostasy, because Islam nullifies what came before it, and repentance nullifies what came before it, as Allah Almighty says: (Say to those who disbelieve, if they cease, what has passed will be forgiven them) [Quran 8:38]. The good deeds you performed before your apostasy are not nullified by your return to Islam. Therefore, you are not required to make up for the prayers and fasts you intentionally omitted, nor for the days you broke your fast in Ramadan due to menstruation.

You must frequently seek forgiveness and perform supererogatory acts of worship, and maintain your prayers and fasts in the future.