Only since 1869 have there consistently been nine justices appointed to the Supreme Court. The U.S. Constitution is silent about how many justices should sit on the Supreme Court.
As reported by AP Fact Check...
TRUMP, on why he’s moving forward with a nomination so close to the Nov. 3 election: “I have a constitutional obligation to put in nine judges — justices.” — remarks Tuesday to reporters.
THE FACTS: To be clear, there is no constitutional requirement to have nine justices on the Supreme Court.
The Constitution, in fact, specifies no size for the Supreme Court, and Congress has the power to change its size.
Over its history, the high court has varied in size from five to 10 justices, depending on the number of judicial circuits in the U.S., according to Russell Wheeler, a visiting fellow at the Brookings Institution and former deputy director of the Federal Judicial Center. He explained that a major duty of the justices until the late 19th century was to try cases in the old circuit courts. Congress decided on nine circuits in the late 1860s.
President Franklin D. Roosevelt pushed to expand the high court in the 1930s in a bid to gain broader judicial support for his New Deal policies, but that effort failed.