Is it unconstitutional for Congress to add SCOTUS seats?

[politicallefty]

Absolutely not. The Constitution grants Congress wide latitude as to the composition of the judicial branch. Article III authorizes "one supreme Court" and "inferior Courts as the Congress may from time to time ordain and establish". I would say the text of the Constitution permits a Supreme Court of simply one Justice. The only constraints are that all judges "shall hold their Offices during good Behaviour" and that their salaries cannot be reduced. Where the Constitution is silent, authority is granted to Congress, the states, or the people (depending on the particular situation).

To be honest, this isn't that different from how the executive branch exists. Most of the executive branch is a creation of law. I don't believe there's anything prohibiting Congress from abolishing most of the executive branch, apart from the President and Vice President. Congress is free to change the executive departments as it sees fit. Cabinet Departments have been created, abolished, and reorganized.

Even Congress itself is a creation of law in terms of size. The Senate's membership is defined by the number of states. Congress has full authority of the creation of new states. That would require the consent of the states in question to reduce the total number. The only stipulation as to the minimum number of the House is that each state must have at least one Member. Technically, the size of the House could be set at 50.