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.”
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.”