Once again @Kamaji The law says NINE justices. Not a word about any "reserves".
It doesn't have to. It says that the Supreme Court - i.e., the active sitting bench - consists of nine justices. Having a "reserve" doesn't break that rule, and isn't prohibited by that rule, so long as the "reserve" is not seating on the court's bench, and is not allowed to opine on cases before the court.
There is nothing in either the Constitution or the statute that requires the existence, vel non, of an actual vacancy on the Court before the current president can nominate a replacement, or before the Senate can hold confirmation hearings. Nothing.
There are no penumbras or emanations that would necessarily prevent that result, either.
And, as
@Cyber Liberty has pointed out, a "dry" confirmation would likely expire at the end of the session of the Senate that made that confirmation, meaning that having a "reserve" is unlikely to create a conflict between the current president and a prior president who may have appointed, and had confirmed, a "reservist" who was never seated because an actual vacancy did not arise during the prior president's term of office.
