Why does the Torah only list five of Binyamin's sons?
Rashi (in Pasuk 24): Because the other five families died out - Refer to 26:13:2:1 - in fulfillment of the prophecy of his mother Rachel, who, in her dying moments, 1 called him "ben Oni" (son of my mourning). 2
Rashi: The prophecy was fulfilled in its entirety by Pilegesh be'Giv'ah, in Shoftim, chapter 20. (Only six hundred men remained. They said "Nigda (was cut off) ha'Yom Shevet Echad mi'Yisrael" (Shoftim 21:6), for there was no one for the remaining men to marry (before they found a solution), and the Shevet would truly die off (Malbim 21:6). (Did they not know that no Shevet can die off? Did they not realize that they can buy Shefachos, free them and marry them? Perhaps they said "Nigda" because less than twenty-ywo thousand remained (refer to 10:36:3:1), or because the majority were killed. - PF).
Who was Achiram, (not mentioned in Vayigash among those who went down to Egypt)?
Rashi: He is synonymous with Echi, who is mentioned there, and the Torah refers to him as 'Achiram' after Yosef, who was his (Binyamin's) brother and his leader.
