Albert Barnes Commentary 2 Kings 10:21

"And Jehu sent through all Israel: and all the worshippers of Baal came, so that there was not a man left that came not. And they came into the house of Baal; and the house of Baal was filled from one end to another." — 2 Kings 10:21 (ASV)

To understand how such a large number of people could fit, we must remember that ancient temples had vast courts surrounding them which could hold many thousands.