Answer:
There is only one Chief Justice at any point in time; the other eight serving with him or her are called Associate Justices. There have been 17 Chief Justices (whether William Cushing held the position or not is a matter of debate; he would be the third in line, making the total 18).
Note
Cushing's role on the US Supreme Court is not a matter of debate. President Washington nominated William Cushing as Chief Justice after the Senate rejected John Rutledge's recess appointment in December 1795. The Senate approved Cushing in January 1796, but he declined the elevation and never became Chief Justice. He continued serving as an Associate Justice on the Supreme Court until his death in September 1810.
Oliver Ellsworth subsequently became third Chief Justice of the United States.