The United States Supreme Court has been made up of nine justices since 1869. The law that makes it so is in “an Act to amend the Judicial System of the United States,” adopted by the United States Congress on April 10, 1869. The citation is 16 Stat. 44, which contains Chapter 22 of laws of the Forty-First Congress.
It took me about fifteen minutes to find a copy of the actual government record.
Authoritative information is readily available, so don’t believe what anyone says just because it sounds like they know what they’re talking about.
The contents of this blog, this web site, and any writings by me that are linked here, are all my personal commentary. None of it is intended to be legal advice for your situation.