Albert Barnes Commentary 1 Chronicles 25:7

"And the number of them, with their brethren that were instructed in singing unto Jehovah, even all that were skilful, was two hundred fourscore and eight." — 1 Chronicles 25:7 (ASV)

With their brethren - that is, "with others of the tribe of Levi." Each son of Asaph, Jeduthun, and Heman was at the head of a band of twelve skilled musicians, consisting partly of his own sons and partly of Levites from other families (1 Chronicles 25:9–31). The 24 band leaders, together with their bands, formed a body of 288 persons (24 x 12 = 288).

In addition to these, we also read of over 3,700 singers. They were probably divided, like the trained musicians, into 24 courses, which would have contained about 155 members each (1 Chronicles 23:5).