C. F. Keil and Franz Delitzsch on Judges 20:17

Jdg 20:17

The forces of the other tribes amounted when numbered to 400,000 men. These numbers (26,000 Benjaminites and 400,000 Israelites) will not appear too great if we consider that the whole of the congregation of Israel took part in the war, with the simple exception of Jabesh in Gilead (Jdg 21:8), and that in the time of Moses the twelve tribes numbered more than 600,000 men of twenty years old and upwards (Num 26), so that not much more than two-thirds of the whole of the fighting men went out to the war.