Q. According to Rebi’s opinion on gemorah Shabbos, we have seven Seforim on the Torah, divided in this parsha, and the Halacha seems to follow him. Why is it then that we always mention five Chumshei Torah and that there are 24 sefarim in Tanach and not 26?

A. The question is asked by the Rashash (Shabbos 115b). Beis Shlomo (ibid.) explains that since there is no separation of four lines by Vayichi Binssoa as there is between the five Chumashim, they are considered as one as is the sefer of Trei Assar, although they are different sefarim, only count as one.
Horav Shlomo Miller’s Shlit’a opinion is similar, and Rebi also agrees that there is basically five sefarim. Yet sefer Bamidbar is divided into three.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a