How can we be sure the Old Testament is accurate if Elijah was the only prophet left?

This morning in church the service was about Elijah with the Baal's prophets. I read this verse and some questions arose which I could not find an answer on. > 1 Kings 18:22 > Then said Elijah unto the people, I, even I only, remain a prophet of the LORD; but Baal's prophets are four hundred and fifty men. The following questions arose: 1. Was the Bible already written in the time of Elijah? 2. If Elijah was the only prophet left, how can we make sure that the Baal's prophets did not corrupt the scriptures? And more in general, how can we make sure that none of the bad kings back in this time did not corrupt the scriptures?