As discussions concerning increasing the number of justices serving on the United States Supreme Court are currently a topic of continuing interest, it might be well to note although the Judiciary Act of 1789 set the number of justices of the Supreme Court at six, a chief justice and five associate justices, in 1807 Congress amended the law and increased the number of justices to seven.
The next change occurred in 1837 when the number of justices was increased to nine while in 1863 the number became 10 justices. In 1866 Congress enacted the Judicial Circuits Act, reducing the number of justices to seven in number, only to raise the number of justices to nine in 1869, which number has survived to date.
Although then President Franklin Delano Roosevelt sought to persuade Congress to enact legislation authorizing the appointment of additional justices to the high court in the event a justice, upon attaining the age 70, declined to resign or retire, and limited the total number of justices serving on the Supreme Court to 15 members, Congress elected not to do so.
See, also, https://publicpersonnellaw.blogspot.com/2018/10/setting-number-of-justices-on-united.html