I'm not pro court packing. But, it was 9 based on there being 9 federal districts. Now there are twelve. I wouldn't do it because Mitch McConnell would figure out a way to add three 28 year old jurists from a QAnon registry.
The number isn't the issue. In a vacuum, nobody cares how many justices there are, and if our justices actually acted like judges rather than like policymakers, I wouldn't care if the Court was expanded. The issue is the motive. If you expand the Court primarily to reverse its judicial philosophy, it's basically the end of the line for the Court. Nobody will take it seriously again, and they shouldn't.