This is a fairly simple story.
A nameless prophet tells King Ahab that God will deliver the "great multitude" of Syrians into his hand that very day, just to let Ahab know that "God is the Lord."
There came a prophet unto Ahab king of Israel, saying, Thus saith the LORD, Hast thou seen all this great multitude? behold, I will deliver it into thine hand this day; and thou shalt know that I am the LORD. 1 Kings 20:13
So Ahab rounded up 232 princes and 7000 "children of Israel" and went off to fight the "great multitude" of Syrians.
Then he numbered the young men of the princes of the provinces, and they were two hundred and thirty two: and after them he numbered all the people, even all the children of Israel, being seven thousand. 20:15
They went out at noon while the Syrian king Benhadad was busy "drinking himself drunk" with the help of 32 other kings.
They went out at noon. But Benhadad was drinking himself drunk in the pavilions, he and the kings, the thirty and two kings that helped him. 20:16
Benhadad must have taken his orders from a different God, though, because he told his soldiers not to kill any prisoners. (The God of the Bible would never allow that. In fact, Ahab will later be killed by God for not killing Benhadad when he takes him prisoner.)
He said, Whether they be come out for peace, take them alive; or whether they be come out for war, take them alive. 20:18
So the Israelites go out and kill lots of Syrians with a "great slaughter."
They slew every one his man: and the Syrians fled; and Israel pursued them ... And the king of Israel ... slew the Syrians with a great slaughter. 20:20-21
And now we all know for sure that God is the Lord.
(Who else would be so proud of killing so many people?)
I guessed 10,000 for this one since it was a "great slaughter."