Jamieson, Fausset and Brown on 1 Chronicles 25:6-7

6-7. All these were under the hands of their father--Asaph had four sons, Jeduthun six, and Heman fourteen, equal to twenty-four; making the musicians with their brethren the singers, an amount of two hundred eighty-eight. For, like the priests and Levites, they were divided into twenty-four courses of twelve men each, equal to two hundred eighty-eight, who served a week in rotation; and these, half of whom officiated every week with a proportionate number of assistants, were skilful and experienced musicians, capable of leading and instructing the general musical corps, which comprised no less than four thousand (1Ch 23:5).

1Ch 25:8-31. Their Division by Lot into Four and Twenty Orders.