It's changed in size over the years...6 to 7 to 9 to 10 to back down to 7, settling on 9 by legislation.
It's been there for something like 150 years.
So I don't think there is anything particular sacred about 9.
At the same time, I recognize that Dems don't have the votes to pack the Supreme Court and this is all just political theater anyways.
I would be more interested in what I call "cascading term limits".....basically pass legislation setting term limits for future Justices of the same term, but having the sitting Justices time out of their present term longest sitting term limiting out first with the next rotating out in say the next 3 years until they're all gone. If you did that, then basically you would have a rotating bench where a new justice would term limit out every three years or so (not saying that the term limit should be 3 years, but because you cascaded the present judges out...whatever the term limit for the new guys is, you would see one term limiting out every three years or so).
If you put the term limit at the same number of justices (9) basically each elected President would get at least one appointment during their term. If you make the term limit longer, there will be gaps where certain Presidents won't get an appointment unless someone retires or resigns, but when the terms started cascading again, those Presidents would get at least on appointment during their term.
There's an argument that such a law wouldn't even require an amendment to the Constitution. I haven't really looked at that issue to have a strong opinion one way or the other.