Albert Barnes Commentary 1 Kings 19:18

"Yet will I leave [me] seven thousand in Israel, all the knees which have not bowed unto Baal, and every mouth which hath not kissed him." — 1 Kings 19:18 (ASV)

Yet I have left me... - This phrase means that seven thousand faithful Israelites will survive all the persecutions of Ahab and Jezebel and will carry on the worship of Yahweh to another generation. Elijah was mistaken in thinking that he was the only one left.

The number is clearly a 'round' number, not an exact count. Furthermore, it is perhaps a mystical or symbolic number. Compare this to Revelation 7:5-8. Of all the symbolic numbers used in Scripture, seven is the most common.

Every mouth which hath not kissed him - Idolaters sometimes kissed their hand as a gesture toward the object of their worship (Job 31:26–27). At other times, they kissed the actual image.