Jamieson, Fausset and Brown on 2 Chronicles 25:5

5. Amaziah ... made captains, &c.--As all who were capable of bearing arms were liable to serve, it was quite natural in making up the muster-roll to class them according to their respective families and to appoint the officers of each corps from the same quarter; so that all the soldiers who formed a regiment were brothers, relatives, friends. Thus the Hebrew troops were closely linked together, and had strong inducements to keep steady in their ranks.

found them three hundred thousand choice men--This was only a fourth part of Jehoshaphat's army (2Ch 17:14-19), showing how sadly the kingdom of Judah had, in the space of eighty-two years, been reduced in population by foreign wars, no less than by internal corruptions. But the full amount of Amaziah's troops may not be here stated.