Two.
Stephen G. Breyer.............(1994 to Present).....................clerked for Justice Arthur Goldberg, 1964-65
John G. Roberts, Jr............(2005 to Present).....................clerked for Justice William H. Rehnquist, 1980-81
John Paul Stevens.............(1975 to 2010).....................clerked for Justice Wiley B. Rutledge, 1947-48