If you could introduce a Constitutional Amendment What would it be

[True Federalist (진정한 연방 주의자)]

A good idea would be to stipulate that a seat gets removed from the judiciary instead of becoming vacant whenever a judge retires or dies, and that seats can only be added to the judiciary via legislation.

Even if you think it a good idea, which I definitely don't,  it doesn't require a constitutional amendment to achieve.  Indeedm it's how the Court was shrunk during the Andrew Johnson administration.