Seven times, according to the Constitution Center.
... the Constitution allows for Congress to decide on how many Justices sit on the Supreme Court’s bench. Article III, Section 1, starts with a broad direction to Congress to establish courts. “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,” it reads.
The Judiciary Act of 1789 established the first Supreme Court, with six Justices. “Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That the supreme court of the United States shall consist of a chief justice and five associate justices, any four of whom shall be a quorum, and shall hold annually at the seat of government two sessions, the one commencing the first Monday of February, and the other the first Monday of August,” the act read.
Since 1789, Congress changed the maximum number of Justices on the Court several times. In 1801, President John Adams and a lame-duck Federalist Congress passed the Judiciary Act of 1801, which reduced the Court to five Justices in an attempt to limit incoming President Thomas Jefferson’s appointments to the high bench. Jefferson and his Republicans soon repealed that act, putting the Court back to six Justices. And in 1807, Jefferson and Congress added a seventh Justice when it added a seventh federal court circuit.
In early 1837, President Andrew Jackson was able to add two additional Justices after Congress again expanded the number of federal circuit court districts. Under different circumstances, Congress created a 10th circuit in 1863 during the Civil War, and it briefly had a 10th Supreme Court Justice. However, Congress after the war passed legislation in 1866 to reduce the Court to seven Justices. That only lasted until 1869, when a new Judiciary Act sponsored by Senator Lyman Trumbull set the number back to nine Justices, with six Justices required at a sitting to form a quorum. President Ulysses S. Grant eventually signed that legislation and nominated William Strong and Joseph Bradley to the newly restored seats.
The number of seats on the Supreme Court is not enshrined in the Constitution. The Founders' generation, in particular, did not regard the number of seats on the Court as fixed. If we're about to seat a Supreme Court justice who considers herself an "originalist," that ought to be relevant at this moment.
Posted with permission from No More Mr. Nice Blog