“Congress had veto-proof majorities then. Are you sure it wouldn’t require consent of the President?”
From the article I read the number of SCOTUS Justices is the EXCLUSIVE province of the two houses of Congress. the President has no role and there isn’t anything in the Constitution that mandates a number (it was originally five and it was increased as the country grew westward and added Appelate Courts). If you find something that says that’s not the case I’d like to see it. The article went into all the times that the number has changed and one was when FDR tried to pack the court so that he could get the New Deal passed. Then, even some of his RAT Party members voted against what he wanted to do ( add three justices ).
https://en.wikipedia.org/wiki/Judicial_Circuits_Act
Not a great source, but it does say Johnson signed the act.
I’m guessing he knew it would be overridden if he vetoed.