Where in the Constitution does it say that?
It is not in the Constitution, it is in the Judiciary Act of 1869. That was the law that set the number of justices at 9. Since it was a law that was passed and signed by the President, it can only be changed by passing legislation to repeal or amend it. I guess if there was no existing law setting the number of justices, and since the Constitution is silent on the matter, the President might be able to appoint justices at will. But since there is an existing law, that has to be changed in order to change the number of justices.