While the Supreme Court was established by Article III of the U.S. Constitution, the size, structure and functioning of the Court was left to Congress to outline in a series of Judiciary Acts. Throughout U.S. history, there have been seven changes to the number of justices sitting on the Supreme Court, the last of which settled at nine justices in 1869 to reflect the nine circuit courts at the time. (There are now thirteen federal circuit courts.)
Remember when the Republicans ran against Biden in 2020, claiming he was going to "expand and pack the court" if he won and the Dems had the House and Senate? Well, the Dems didn't do that. They also didn't make Puerto Rico and Guam states, nor did they award Senators to D.C.
Funny how the Republicans scare the populace with things the Dems won't do (Death Panels, "kill your grandma" and "round up all the guns and take them away"), while failing to actually do the things they claim they WILL do themselves (build a wall that Mexico will pay for, "Repeal and Replace Obamacare").