Is It Mandatory To Have Nine U.S.Supreme Court Justices ?

Just the other day someone asked me if the U.S. Constitution requires nine Supreme Court Justices. I did not know the answer so I looked it up. The short answer is the U.S. Constitution does not state the precise number of justices. The constitution gave Congress the power to decide the number of justice. Since The Judiciary Act of 1869, the number has been set at nine with no change. Is the Senate violating the Judiciary Act of 1869 by not acting on the vacancy created by Justice Scalia's passing ? Is there a time limit or can the senate refuse to act on the vacancy ? I guess I will read the act in the next few days.