No, the Constitution does not require 9 Supreme Court justices

The Constitution leaves the number of justices to the discretion of Congress, and there have not always been 9

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Citation From the September 21, 2020, edition of MSNBC's Andrea Mitchell Reports

ANDREA MITCHELL (ANCHOR): Sen. Schumer, on your caucus call yesterday, threatened that everything is on the table if the Republicans do push this through. So is that wise to threaten everything, meaning the end of the filibuster, you know, packing the court, changing constitutional -- you know, the constitutional number of nine justices? Is that wise right before an election to make those threats? Because you could lose those Republican moderate, suburban voters, many women who might otherwise be voting for Joe Biden over Donald Trump. 
 

According to the National Constitution Center, “the Constitution allows for Congress to decide on how many Justices sit on the Supreme Court’s bench.” The Supreme Court has had as few as five and as many as 10 justices, settling on the current nine-member court in 1869.