1 Chronicles 23:4-5, 28-32

Of which, twenty and four thousand were to set forward the work of the house of the LORD; and six thousand were officers and judges: moreover four thousand were porters; and four thousand praised the LORD with the instruments which I made, said David, to praise therewith.

because their office was to wait on the sons of Aaron for the service of the house of the LORD, in the courts, and in the chambers, and in the purifying of all holy things, and the work of the service of the house of God; both for the shewbread, and for the fine flour for meat offering, and for the unleavened cakes, and for that which is baked in the pan, and for that which is fried, and for all manner of measure and size; and to stand every morning to thank and praise the LORD, and likewise at even; and to offer all burnt sacrifices unto the LORD in the sabbaths, in the new moons, and on the set feasts, by number, according to the order commanded unto them, continually before the LORD: and that they should keep the charge of the tabernacle of the congregation, and the charge of the holy place, and the charge of the sons of Aaron their brethren, in the service of the house of the LORD.