Ok, not a downvoter, but I’ll bite. A system with more than one party is more democratic than a system with a single party. Are you asking which of the two major parties in the US is the “second” party, making the US more democratic than if there were a single party? If so, I see why no one is answering. If not, perhaps you could clarify your question?
I’m not sure why you believe this is false? From https://www.supremecourt.gov/about/faq_general.aspx : “Who decides how many Justices are on the Court?: The Constitution places the power to determine the number of Justices in the hands of Congress. The first Judiciary Act, passed in 1789, set the number of Justices at six, one Chief Justice and five Associates. Over the years Congress has passed various acts to change this number, fluctuating from a low of five to a high of ten. The Judiciary Act of 1869 fixed the number of Justices at nine and no subsequent change to the number of Justices has occurred.”