How Many Supreme Court Justices Are There?
There are nine members of the Supreme Court, and that number has gone unchanged since 1869. The number and length of the appointments are set by statute, and the U.S. Congress has the ability to change that number. In the past, changing that number was one of the tools that members of Congress used to rein in a president they didn't like.
http://www.thoughtco.com/how-many-supreme-court-justices-are-there-104778
...appears that it can be done by normal legislative procedure. House and Senate passes it and pres either signs it or vetoes it. If he does, they have the ability to over ride the veto.

Liberals are like a "Slinky". Totally useless, but somehow ya can't help but smile when you see one tumble down a flight of stairs!