How can we reconcile the preservation of the Prophetic Sunnah with the statement that many hadiths have not reached us, citing the words of Sheikh al-Islam: "And indeed, much of what reached them and was authenticated by them may not reach us, or may only reach us through an interrupted chain of transmission," especially with the existence of lost compilations like Musnad Baqi ibn Makhlad, and the claim that Imam Ahmad memorized a million hadiths while the existing books of Sunnah do not amount to one hundred thousand hadiths?

Summary: The Sunnah (definition) is preserved just like the Holy Quran, because Allah Almighty says: {Indeed, We have sent down the Dhikr (definition), and indeed, We will be its Guardian} (Quran 15:9). Sheikh al-Islam Ibn Taymiyyah explained that "Dhikr" includes both the Book (Quran) and the Sunnah.

As for what Sheikh al-Islam Ibn Taymiyyah mentioned regarding the possibility that some hadiths may not have reached us, his intention was that some hadiths might be unknown to certain scholars or later imams, not that they were unknown to the entire Ummah. The early imams had more knowledge of the Sunnah because their breasts were their books. Sheikh al-Islam Ibn Taymiyyah did not claim that the Sunnah was confined to specific written collections.

As for their statement that Imam Ahmad memorized a million hadiths, this refers to hadiths with their various chains of narration and isnads, in addition to the narrations of the Companions and the fatwas of the Tabi'in (successors).

And Allah knows best.