Well, not necessarily. Lifetime appointments can be reduced to say 15 years or 18 years for new judges, doubt it is in the constitution. And, if the numbers can be increased by law so can it be decreased by law. Stacking is also not on constitution.
The other point is what happened to the lower federal courts since moscow mitch took over if not stacking, limited obama and spent most of time stacking under trumpster.
And, I think the whole congress has to pass an increase to the sups. Hard to do if it is split.