1 Chronicles 27:1
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.
The arrangement was an organized rotation of men fit to bear arms in the king's service. They were divided into twelve courses of twenty-four thousand men, with a captain over each course. Each course came on duty for one month of the year and then gave place to the next, so that service came round month by month through all twelve months. By this monthly rotation responsibility and service were spread broadly, so that no single person or class carried an undue burden.