Jamieson, Fausset and Brown on 1 Chronicles 7:5

5. fourscore and seven thousand--exclusive of the 58,600 men which the Tola branch had produced (1Ch 7:24), so that in the days of David the tribe would have contained a population of 45,600. This large increase was owing to the practice of polygamy, as well as the fruitfulness of the women. A plurality of wives, though tolerated among the Hebrews, was confined chiefly to the great and wealthy; but it seems to have been generally esteemed a privilege by the tribe of Issachar, "for they had many wives and sons" [1Ch 7:4].

1Ch 7:6-12. Of Benjamin.