Albert Barnes Commentary Jeremiah 33:21-22

"then may also my covenant be broken with David my servant, that he shall not have a son to reign upon his throne; and with the Levites the priests, my ministers. 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." — Jeremiah 33:21-22 (ASV)

This promise also has been fulfilled not literally but spiritually, for only in this sense have the seed of David and the Levites been multiplied.