John Gill Commentary Numbers 31:4

"Of every tribe a thousand, throughout all the tribes of Israel, shall ye send to the war." — Numbers 31:4 (ASV)

Of every tribe a thousand So that the whole number of those that were armed were 12,000 as after given:

throughout all the tribes of Israel ; this is observed, as Jarchi thinks, to comprehend the tribe of Levi, which in some cases was left out of the account:

shall you send to the war ; to fight with Midian.