Is the US Supreme Court the only court established by the Constitution?

No. The framers of the Constitution anticipated there would be a need for a more complex court system, but they gave power to Congress to determine what form the federal judiciary would take.

Article III, Section 1, mandates the creation of a Supreme Court and inferior 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. The judges, both of the supreme and inferior courts, shall hold their offices during good behaviour, and shall, at stated times, receive for their services, a compensation, which shall not be diminished during their continuance in office.