Albert Barnes Commentary Exodus 18:21

"Moreover thou shalt provide out of all the people able men, such as fear God, men of truth, hating unjust gain; and place such over them, to be rulers of thousands, rulers of hundreds, rulers of fifties, and rulers of tens:" — Exodus 18:21 (ASV)

Able men - The qualifications are remarkably complete: ability, piety, truthfulness, and unselfishness. From (Deuteronomy 1:13), it appears that Moses left the selection of the persons to the people, an example followed by the Apostles .

Rulers of thousands ... - The numbers appear to be conventional, corresponding nearly, but not exactly, to the military, or civil divisions of the people: the largest division (1,000) is used as an equivalent of a gens under one head (Numbers 1:16; Numbers 10:4; Joshua 22:14).

The word “rulers,” sometimes translated as “princes,” is general, including all ranks of officials placed in command. The same word is used regularly on Egyptian monuments of the time of Moses.