A quorum of justices to hear and decide a case is six. If, through recusals or vacancies, fewer than six justices can participate in a case, and a majority of qualified justices determines that the case can not be heard in the next term, then the decision of the court below is affirmed as if the Court had been equally divided on the case. An exception exists when this situation arises in one of the now-rare cases brought directly to the Supreme Court on appeal from a United States District Court; in this situation, the case is referred to the U.S. Court of Appeals for the corresponding circuit for a final decision there by either the Court of Appeals sitting en banc, or a panel consisting of the three most senior active circuit judges.

A quorum for a decision in the supreme court is? Okie... think carefully first, then make a decision:
The relevant information to answer the above question is: A quorum of justices to hear and decide a case is six. So, the answer is six.