Where is 9 justices enumerated in the Constitution? SCOTUS started out with 6. An even number intentionally so when there was a tie the decision would default back closer to the people and the lower court decision.
IIRC, the number of SCOTUS justices as being nine was fixed by law in 1869.
In 1937 FDR tried to `pack’ the Supreme Court by having legislation introduced that would authorize one additional justice for each current justice over age 70. This would have meant four more. It failed by a whisker.