Albert Barnes Commentary 1 Kings 20:13

"And, behold, a prophet came near unto Ahab king of Israel, and said, Thus saith Jehovah, Hast thou seen all this great multitude? behold, I will deliver it into thy hand this day; and thou shalt know that I am Jehovah." — 1 Kings 20:13 (ASV)

The rabbinical commentators conjecture that this prophet was Micaiah, the son of Imlah, who is mentioned below in 1 Kings 22:8.

Hast thou seen all this great multitude?—The boast of Ben-hadad in 1 Kings 20:10 was not without a basis of truth; his force seems to have exceeded 130,000 (compare 1 Kings 20:25, 29–30). In his wars with the Assyrians, we find him sometimes at the head of 100,000 men.