How many Supreme Court justices are there, and why nine?

The Supreme Court has nine justices, one chief justice and eight associates. The Constitution establishes the Court but is silent on its size, leaving that to Congress, which set it at six in the Judiciary Act of 1789 and has changed it repeatedly since, to five, seven, nine, ten, back to seven, and finally to nine in 1869, where it has remained. Several of those changes were nakedly political, timed to expand or restrict a particular president's appointment power, which is worth knowing because it establishes that the number is a statutory choice with a partisan history rather than a constitutional fixture.

The most famous attempt to change it failed. In 1937, Franklin Roosevelt proposed legislation that would have let him appoint additional justices, widely understood as a response to the Court striking down New Deal programs, and the plan collapsed under bipartisan opposition even in his own party, cementing nine as a strong norm. Proposals to expand the Court have resurfaced periodically since, including in recent years, and they remain constitutionally permissible and politically contentious for the same reason they always were. Justices serve during good behavior, effectively for life absent impeachment or retirement, which is why each vacancy carries such weight and why proposals for fixed terms appear alongside proposals about size.