How many did God kill in Elijah's prayer contest?

At least 450

(It may have been 850 since 1 Kings 18:19 mentions 450 prophets of Baal and 400 prophets of the groves.)
Now therefore send, and gather to me all Israel unto mount Carmel, and the prophets of Baal four hundred and fifty, and the prophets of the groves four hundred, which eat at Jezebel's table. ...

And Elijah said unto them, Take the prophets of Baal; let not one of them escape. And they took them: and Elijah brought them down to the brook Kishon, and slew them there. 1 Kings 18:19-40