the judiciary act of 1869 sets the number at nine.Exactly, there has to something preventing it
Judiciary Act of 1869 - Wikipedia
but this is just legislation. Congress could pass legislation to expand the court to whatever number they want because the Constitution does not specify this.
i think republicans haven’t done this because they don’t want to cross a rubicon where when the next party is in power, they just appoint more justices etc. under normal circumstances this would be true of the democratic party as well, but i think republicans have already crossed the rubicon because of how they handled the garland nomination and now this coney barrett nomination