When the Torah refers to itself, it refers to itself in the singular. There are numerous verses, notably in Devarim. One notable example Devarim 31:16 (Sefaria English translation:)
"לָקֹחַ אֵת סֵפֶר הַתּוֹרָה הַזֶּה וְשַׂמְתֶּם אֹתוֹ מִצַּד אֲרוֹן בְּרִית־ה' אֱלֹקיכֶם וְהָיָה־שָׁם בְּךָ לְעֵד׃"
Take this book of Teaching and place it beside the Ark of the Covenant of the LORD your God, and let it remain there as a witness against you.
Yet, we see that there are five sefarim (books) and on parchment, we denote the separate books by leaving several lines of spacing between the end of one book and the start of the next.
I notice that Rash"i as well as the Gemarah refer to some of the separate books by names such as "Torat Kohanim" (Vayikra) and "Mishneh Torah" (for Devarim).
When did the concept of separating the Torah into Five books begin? And, why was there a requirement to write a Torah into five books? Is that Misinai (from Sinai)? It doesn't appear to be this way from the Torah's verbage. Even it is Misnai, why separate it into five books?