You still need consent of the Senate to put someone on the court.They have to hold their office during good behaviour. And the Supreme Court is distinct from whatever inferior courts.
Yeah, the easiest thing is to fuck with the numbers. Like appointing all the Circuit Appeal Court judges "Associate Justices of the USSC". Or something like that. And then you'd have to figure out how to sub in those judges for the current idiots.
But yes, Congress could just make it 81 judges if they wanted, with groups of 9 appointed at random to hear specific cases.