Was the size of the Supreme Court originally proportional to the number of circuit courts?
Although the Constitution established the Supreme Court, it was the Judiciary Act of 1789 that laid out the Court's organization, including that there would be six justices. The Act put the Supreme Court at the top of a three-tier system which included three circuit courts based on geographical regions. Originally, two justices were required to preside over each circuit court twice a year.
The Judiciary Act of 1802 increased the number of circuit courts to six, the same as the number of Supreme Court justices. As the population of the U.S. grew, the number of circuit courts increased accordingly through the first half of the 19th century.
In 1869, Congress set the number of justices at nine, one for each circuit. Since then, the number of circuit courts has increased to 13, but the number of seats on the Supreme Court has remained at nine.