Adam Clarke on Romans 11:4

But what saith the answer of God - The answer which God made assured him that there were seven thousand, that is, several or many thousands; for so we must understand the word seven, a certain for an uncertain number. These had continued faithful to God; but, because of Jezebel's persecution, they were obliged to conceal their attachment to the true religion; and God, in his providence, preserved them from her sanguinary rage.

Who have not bowed the knee - Baal was the god of Jezebel; or, in other words, his worship was then the worship of the state; but there were several thousands of pious Israelites who had not acknowledged this idol, and did not partake in the idolatrous worship.