Charles Ellicott Commentary Numbers 31:30

"And of the children of Israel`s half, thou shalt take one drawn out of every fifty, of the persons, of the oxen, of the asses, and of the flocks, [even] of all the cattle, and give them unto the Levites, that keep the charge of the tabernacle of Jehovah." — Numbers 31:30 (ASV)

One portion of fifty. —The Levites were much more numerous than the priests, and consequently it was ordered that they should have two percent of the spoil which fell to the congregation, whereas the priests had only one-fifth of one percent of a like amount. The portion of the Levites, therefore, was 320 maidens, 6,750 sheep and goats, 720 oxen, and 610 asses.