This promise also has been not literally, but spiritually fulfilled, for in this sense only have the seed of David and the Levites been multiplied.
Related Commentaries of Jeremiah 33:21
Jeremiah 33:21-22
21 Then may also my covenant be broken with David my servant, that he should not have a son to reign upon his throne; and with the Levites the priests, my ministers.
22 As the host of heaven cannot be numbered, neither the sand of the sea measured: so will I multiply the seed of David my servant, and the Levites that minister unto me.