Albert Barnes Commentary 2 Chronicles 14:9

"And there came out against them Zerah the Ethiopian with an army of a thousand thousand, and three hundred chariots; and he came unto Mareshah." — 2 Chronicles 14:9 (ASV)

Zerah the Ethiopian is probably Usarken (Osorkon) II, the third king of Egypt after Shishak, according to Egyptian monuments. Osorkon II may have been an Ethiopian by birth, as he was the son-in-law—not the son—of the preceding monarch and reigned by right of his wife. The purpose of the expedition was to bring Judea once more under the Egyptian yoke.

An host of a thousand thousand - This is the largest assembled army mentioned in Scripture, but it does not exceed the known numbers of other ancient Eastern armies. Darius Codomannus brought a force of 1,040,000 to the field at Arbela; Xerxes crossed into Greece with certainly more than a million combatants.