One of fifty — Whereas the former part was one of five hundred; the reason of the difference is, partly, because this was taken out of the peoples portion, whose hazards being less than the others, their gains also in all reason were to be less: partly because this was to be distributed into more hands, the Levites being now numerous, whereas the priests were but few.
Related Commentaries of Numbers 31:30
Numbers 31:30
30 And of the children of Israel's half, thou shalt take one portion of fifty, of the persons, of the beeves, of the asses, and of the flocks,c of all manner of beasts, and give them unto the Levites, which keep the charge of the tabernacle of the LORD.