The size of the Supreme Court has varied from 6-12 people, over time. There is no defined number of justices, either max or min.
The next Democratic president and senate could simply sign on 4 new justices that were heavy lefties, and, say, 30 years old, and the entire thing could be changed. Forever.
Then the Repubs win, and make the Supreme Court 30 members strong, and sign on 18 new justices. And with their total abdication of moral code or dignity, all cards are on the table, from now for ever.