How many? I'm not sure. The late Menachem Schneerson, rebbe of the Chabad Lubavitch sect of chasidism, was of this line. Prior to his death, some of his followers thought that he might be the messiah. There are other families which have preserved knowledge of their descent, just as the kohenim are descended from the priestly families of the temple period, even though it has been nearly 2000 years since the destruction of the second temple.