There IS a law stipulating the number of justices. The number is not set by the constitution, which I think is where you got the idea. Changing the law that sets the number would require an act of Congress, which means a 2/3rds majority in the Senate because of the filibuster rule. 50% could overturn the filibuster rule and then stack the court, but 2 right leaning Democrats from Republican states refuse to overturn the filibuster rule, so it’s just not possible unless more progressives are in the Senate.
Getting a more progressive Senate is hard because it’s not proportional representation. North Dakota with a population under 1 million gets the same number of Senators as California with 40 million. Rural voters are wildly over-represented in the Senate.