According to the Hebrew text Benjamin has ten sons. It is, of course, impossible that he became the father of ten sons between his return to Canaan in chapter 45 and the departure of Jacob for Egypt in chapter 46. The Septuagint variation is even more impossible to fit into the time frame of the Joseph story. However, we must translate verse 21 as the text stands.

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .