From How Democracies Die:
Jefferson's victory did not put an end to the intense partisan acrimony. The lame-duck Federalist Congress reduced the size of the Supreme Court from six to five to limit Jefferson's influence over the Court. With its new majority, the Republican Congress repealed the move, and a few years later, it expanded the Court to seven to give Jefferson another appointment.
This was the
Midnight Judges Act. It never occurred to me (and maybe modern congress) that congress can change the number of SCOTUS judges.