Even though 2 Nephi 5:6 gives a good overview of the lineages who supported Nephi, as discussed above regarding that verse, this is the first time in the Book of Mormon where it is stated that Lehi’s colony was in fact fully divided into seven distinct tribes. These tribal designations are repeated in 4 Nephi 1:36–37, Mormon 1:8, and even in D&C 3:16–18. Seven was a sacred number in ancient Israel, which may explain, at least in part, why these seven tribes lasted for approximately 1,000 years of Nephite and Lamanite history. Interestingly, seven was also a sacred number among ancient Mesoamerican societies, and various legends from that region of the world depict their peoples as having emerged from seven caves or lineages. While no definitive connection can be made between these legends and the Book of Mormon, the relationship is certainly intriguing and may point to a shared historical setting.
Book of Mormon Central, "Why Did Lehi Divide His People into Seven Tribes? (Jacob 1:13)," KnoWhy 319 (May 29, 2017).
Diane E. Wirth, "Revisiting the Seven Lineages of the Book of Mormon and the Seven Tribes of Mesoamerica," BYU Studies Quarterly 52, no. 4 (2013): 77–88.