A thousand of every tribe, to prevent emulations or divisions. God would send no more, though it is apparent that the Midianites were numerous and strong, because he would try and exercise their trust in him, and because he would hereby give them an earnest of their Canaanitish conquests.
Related Commentaries of Numbers 31:5
Numbers 31:5
5 So there were delivered out of the thousands of Israel, a thousand of every tribe, twelve thousand armed for war.