This is the list of the Israelites—the heads of families, the commanders of thousands and of hundreds, and their officers who served the king in every matter concerning the divisions on rotating military duty each month throughout the year. There were 24,000 men in each division:
We have here an account of the regulation of the militia of the kingdom. David was himself a man of war, and had done great things with the sword; he had brought into the field great armies. Now here we are told how he marshalled them when God had given him rest from all his enemies.
Commenting on 1 Chronicles 27:1-15
Now the children of Israel after their number,.... Not the whole body of the people, but the militia of the nation; for after the account of the division of the priests and Levites into courses, follows an account of the militia of the nation, being divided also into monthly courses; which, though done in the beginning of David's reign, as Kimchi and Jarchi observe, yet...
Now the children of Israel after their number, [to wit], the chief fathers and captains of thousands and hundreds, and their officers that served the king in any matter of the courses, which came in and went out month by month throughout all the months of the year, of every course [were] twenty and four thousand.