John Gill 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)

So the number of them, with their brethren: Who each of them had twelve with them, as after reckoned. that were instructed in the songs of the Lord: taught how to sing them. [even] all that were cunning: expert and well skilled in the science of singing. was two hundred and eighty eight: for there being twenty four classes answerable to those of the priests, and twelve in each class, twelve times twenty four makes two hundred and eighty eight.