Answer: Nine.
Byron R. White clerked for Chief Justice Fred Vinson during the 1946 Term.
William H. Rehnquist clerked for Justice Robert H. Jackson during the 1952 Term.
John Paul Stevens clerked for Justice Wiley B. Rutledge during the 1947 Term.
Stephen G. Breyer clerked for Justice Arthur J. Goldberg during the 1964 Term.
John G. Roberts, Jr., clerked for Justice William H. Rehnquist during the 1980 Term.
Elena Kagan clerked for Justice Thurgood Marshall during the 1987 Term.
Neil M. Gorsuch clerked for then-retired Justice Byron R. White and Justice Anthony M. Kennedy during the 1993 Term.
Brett M. Kavanaugh clerked for Justice Anthony M. Kennedy during the 1993 Term.
Amy Coney Barrett clerked for Justice Antonin Scalia during the 1998 Term.