Gonna have to give you a Joe Biden "come on, man" yourself.
It has changed 7 times in 230+ years. Clearly it is something that sets a strong norm for multiple generations at a time.
2 of those 7 times were completely political and then adjusted. In 1866, Congress passed the Judicial Circuits Act, which shrank the number of justices back down to seven and prevented President Andrew Johnson from appointing anyone new to the court. Three years later, in 1869, Congress raised the number of justices to nine, where it has stood ever since.
As you can see the last two times was purely political, was corrected and has remained exactly the same for more than 150 years. It is clear the "norm" is where it is now. Ruth Bader Ginsburg, herself, was interviewed a number of years ago where she opined that 9 was exactly where it should be and that is why she believed it had not been monkeyed with for 125+ years at the time. She also opined back then that packing the supreme court should not happen, FWIW. She said it would turn to be as we are now discussing and that the judicial branch should never be a partisan extension of another branch.