The first five books of our Old Testament have two names, a Western one, Pentateuch, and a Jewish one, Torah. Sometimes they are referred to as the Books of Moses or the Law. The word Torah means “teaching.” The Greek name for the Torah is the Pentateuch which means “five-volume book,” not “five books.” The Latin Vulgate translator Jerome popularized the term Pentateuchus probably as a translation of a Hebrew title used still today meaning, “five-fifths of the Torah.” The Bible also understands Genesis through Deuteronomy as Joshua 1:8 calls it the “Book of the Torah.” We see the same idea in Ezra 6:18; Nehemiah 13:1-2; 2 Chronicles 25:4; 35:12, and Mark 12:26.
Further evidence is in the five-fold book itself. We see a travelogue outline across the Torah, eg., the Sinai outline, Exodus 19:1 going into Sinai and Numbers 10:12 going out. The Damascus Document from Qumran and the Letter of Aristeas speaks of books or scrolls of the law. Exodus, Leviticus, and Numbers have a unity suggesting they were written that way on purpose. (Exodus 19:1-Numbers 10:28). Ordination of priests in Leviticus 8 is connected with rules for ordination in Exodus 29. The religious rules in Exodus 25-31, 35-40 continue into Leviticus and Numbers, and locations are the same in Numbers 6:13 and Deuteronomy 1:1-5.
So why was divided into five parts? Probably practicality. Perhaps translation into Greek with the addition of vowels caused the division. In fact, our names for the sections come from Greek tradition and are different from the Hebrew names. Perhaps the size and weight of the scrolls dictated a division into five sections. If put all together, the entire Torah would be 99 feet long! Imagine having to roll to Leviticus. You’d have to get to synagogue at least an hour early to turn to the passage! Each section in fact is named after the first few words of that part.