Albert Barnes Commentary 2 Chronicles 25:5

"Moreover Amaziah gathered Judah together, and ordered them according to their fathers` houses, under captains of thousands and captains of hundreds, even all Judah and Benjamin: and he numbered them from twenty years old and upward, and found them three hundred thousand chosen men, able to go forth to war, that could handle spear and shield." — 2 Chronicles 25:5 (ASV)

Three hundred thousand - Asa’s army had been nearly twice as numerous, amounting to 580,000 (2 Chronicles 14:8). The diminution was due, in part, to wars (2 Chronicles 21:8, 16; 24:23–24) and, in part, to the general decadence of the kingdom.