The number of judges on the Supreme Court is set by congress, not the constitution. The number of judges has varied from 6 to 10 over time. We have had nine justices sine the Judiciary Act of 1869.
During the Great Depression, when the Court ruled that some of President Franklin Delano Roosevelt’s New Deal legislation was unconstitutional, he proposed adding new justices to balance the Court. His own party opposed that, however, and the plan was defeated.