http://en.wikipedia.org/wiki/Supreme_Court_of_the_United_StatesCongress determines the number of justices on the Court.
There have been nine justices on the Court since 1869. There were originally six until 1807 when 7th justice was added. In 1837 an 8th and 9th were added with a 10th in 1863. The Judicial Circuits Act of 1866 called for the removal of three seats as justices retired. One was removed in 1866 and a second in 1867. Before the third could be removed, Congress passed the Circuit Judges Act of 1869 which raised the number of seats back to nine. Since 1869, the Court has been kept at nine both for political reasons as well as practical necessity. Subsequent attempts to change the number of justices have since been rejected.
The last notable attempt to alter the number of justices was on February 5, 1937 when President Franklin Delano Roosevelt proposed an increase in the size of the Court to fifteen justices, one additional seat for each justice over age 70, to deal with a Court unwelcoming of Roosevelt's New Deal legislation. Many politicians at the time and historians since scorned this plan to "pack the court". The proposal failed on July 22 when the
United States Senate voted against it. Several years ago, the Supreme Court delivered a highly controversial 5-4 decision in Bush v. Gore, 531 U.S. 98 (2000), that ended weeks of bitter legal maneuvering between lower courts following the 2000 presidential election.
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With a Repuke majority in Congress, Bush COULD change the number of Justices. I wouldn't put it past him to do something this drastic. It would not require a Constitutional amendment.