Article III, Section 1, of the U.S. Constitution says that "the judicial Power of the United States shall be vested in one supreme Court, and in such inferior courts as the Congress may from time to time ordain and establish." The document says nothing about how many justices should actually sit on the Supreme Court, leaving that rather important decision entirely in the hands of Congress.
For advocates of court packing, or court expansion, or whatever other label the idea of adding new justices to the Court might happen to be going by nowadays, this constitutional language is a very handy ally to call upon and legal authority to cite. Whether you are in favor of court packing, opposed to it, or simply agnostic about the concept, there is no doubt about its constitutionality.
