History of the Supreme Court of the United States
The Supreme Court of the United States is the only court specifically established by the Constitution of the United States, implemented in 1789; under the Judiciary Act of 1789, the Court was to be composed of six members—though the number of justices has been nine in its history, this number is set by Congress, not the Constitution. The court convened for the first time on February 2, 1790.
Source: Wikipedia — History of the Supreme Court of the United States (CC BY-SA 4.0)