Congress has the sole authority to decide how many Justices sit on the Supreme Court. The US Constitution does not stipulate the size of the Supreme Court.
Currently Title 28 of the United States Code Section 1 provides for 1 Chief Justice and 8 Associate Justices. Congress has changed the number of justices several times, the last time being in the Judiciary Act of 1869.
The Judiciary Act of 1789 created the first Supreme Court with six members, a Chief Justice and 5 Associate Justices. Congress adjusted the size of the Court a number of times through the during the 19th-century.
- Judiciary Act of 1789: Court size 6
- Judiciary Act of 1801: Court size, 5
- Repeal Act of 1802: Court size, 6
- Seventh Circuit Act of 1807: Court size, 7
- Judiciary Act of 1837: Court size, 9
- Tenth Circuit Act of 1863: Court size, 10
- Judicial Circuit Act of 1866: Court size, 7
- Habeas Corpus Act of 1867: Court size, 8
- Judiciary Act of 1869: Court size, 9
After the election of President Ulysses S. Grant, Congress passed the Judiciary Act of 1869, which set the Court's membership at nine. This number has remained the same ever since.
In 1937, Franklin D. Roosevelt attempted unsuccessfully to expand the membership of the court to gain support on the Court for his New Deal programs. He proposed adding one justice to the Supreme Court for every member over 70.5 years of age, with the potential of adding as many as six additional justices, for a total of 15. Congress refused to pass Roosevelt's legislation; however, the President had an opportunity to nominate eight justices* to vacancies that occurred during his terms of office, which created a court more receptive to his ideas.