John Gill Commentary 2 Chronicles 17:19

"These were they that waited on the king, besides those whom the king put in the fortified cities throughout all Judah." — 2 Chronicles 17:19 (ASV)

These waited on the king Not this vast number of men at one time, but in course, as David's military men waited on him, (1 Chronicles 27:1–15) or each of these captains, with a proper number of men under them detached from the body; and they were all ready to obey the king's orders whenever he should have occasion for them:

besides those whom the king put into the fenced cities throughout Judah; and all together, the militia of the kingdom was very numerous and formidable.