I don’t think there is anything in the Constitution stating how many Justices sit on the Supreme Court or that it has to be a lifetime term.
I states the appointment is a lifetime. Please read the constitution.
Article Three, Section 1 Justices “shall hold their offices during good behavior”, which is understood to mean that they may serve for the remainder of their lives, until death; the only way justices can be removed from office is by Congress via the impeachment process but that ha snot been tested.
Correct as to the first, but not as to the second. The number of Supreme Court Justices is set by federal statute. See 28 U.S.C. sec. 1:
"The Supreme Court of the United States shall consist of a Chief Justice of the United States and eight associate justices, any six of whom shall constitute a quorum."
But with respect to something like "term limits" for Supreme Court Justices (or any federal judge, for that matter), Article III, Section 1 of the Constitution itself specifies:
"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."
The highlighted portion here has been construed to mean that Supreme Court Justices and Article III federal judges are to hold "lifetime appointments," and can be removed from their position only through impeachment.
As an aside, note that Article III, Section 1 indicates that Congress itself establishes the balance of the federal judiciary. On the face of Article III, only the Supreme Court has an independent, constitutionally-grounded status. Congress could by statute abolish the entirety of the federal judiciary, save for the Supreme Court, if it wished.