John Gill Commentary Numbers 26:5

"Reuben, the first-born of Israel; the sons of Reuben: [of] Hanoch, the family of the Hanochites; of Pallu, the family of the Palluites;" — Numbers 26:5 (ASV)

Reuben, the eldest son of Israel
the number of his tribe is taken first on that account; there were four families that descended from him, the Hanochite, Palluite, Hesronite, and Carmite, and the number of men from twenty years old and upwards, fit for war, were 43,730; so that, since the last numbering, this tribe was decreased 2770; which may in part be accounted for by the families of Dathan and Abiram being cut off for their rebellion, who were this tribe, and whose affair is briefly related in the following verses, and it may be to point out this unto us.