"What happens if a SC justice gets incapacitated? Goes on a ventilator or something and cannot function? Is there a process for replacing that justice or do we have to wait for her/him to die? Thanks."
We'll just lie to Greer and tell him he's next in line for the job. He'll take care of the rest.
There's no limit to how many judges may be placed on the Supreme court; there's only historical precedent. If a judge on the USSC was incapacitated substantially enough, there's no reason that the President and the Senate could not place another person on the court. The only possible drawback could be political fallout, if the public perceived this as an attempt to "pack the court."