That was like 150 years ago. It had been 10 before that. So I don't think "the number of circuit courts" has any particular signifigance. The changes in number have always been related to political concerns, not practical ones in the administration of circuit courts.
Many changes were not related to political concerns and the size of the court was directly tied to the number of circuit courts.
The Supreme Court was originally 6, which matched the 6 circuits. Each Supreme Court Justice is assigned to a circuit (more than one now). Until the late 1800s, Supreme Court Justices were required to ride their assigned circuit to hear appeals. Justice were traditionally appointed with judges residing in the area covered by the circuit to which they would be assigned.
In 1801, the Supreme Court was reduced to five with changes to how appeals would be handled in the circuits. But that was repealed 1802 and the Supreme Court restored to 6 without the 1801 changes having any affect. That was all political with the lame-duck Adams and the Federalists try to establish judicial power before Jefferson took over, and Jefferson subsequently trying to undo those actions (which led to Marbury v. Madison).
When the number of circuits was increased to 7, the Supreme Court was increased to 7. When increased to 9, the Supreme Court was increased to 9.
When increased to 10, the Supreme Court was increased to 10. This abolished the short-lived California circuit.
Republican Lincoln ran with Democrat Johnson as his Vice President. After Lincoln was assassinated, the Republican controlled Congress didn't like the way Johnson was handling reconstruction. When a Supreme Court seat opened, they restructured the circuits into 9 circuits. They considered reducing the seats to 9 (to take away a Johnson appointment and to resolve the issue of a possible tie vote), but they decided to further further reduced it 7 to keep Johnson from getting any appointments (and in part from Justice Chase trying to justify a raise for himself).
After Grant was elected, Congress restored the number of seats to 9 to match the number of circuits.
They circuits got replaced by the district courts and the courts of appeals, which had the appellant authority of a Supreme Court Justice. Eventually Supreme Court Justices were no longer required to ride the circuit at all.
By the time new districts were added, Supreme Court Justices were no longer so closely tied to the circuits and no new Supreme Court seats were added and it has remained at 9 ever since.
Other than the reduction to 7 during the Johnson administration, the number of Justices was always tied directly to the number of circuits. But it stopped being increased when the district and appellant courts took over much of the circuit work that Justices had previously done.