Why is there an odd number of justices on the US Supreme Court?

One possible reason (amongst many presumably) are that any vote taken by the justices can never result in a tie - a decision will always be made one way or the other.

In 1869, Congress passed a law that fixed the number of Supreme Court justices at 9. The US Constitution creates "one supreme court" but does not specify how many justices there should be. That is up to Congress to decide. The number of justices is an usually an odd number so that there would be no tie votes. Why there are 9 though is a matter of practicality based on the workload of the Supreme Court. The first Supreme Court had 6 justices, then it changed to 5, then to 7 then to 9 then to 10 then to 7 again and then back to 9.

The original U.S. Supreme Court had only six Justices; that number has changed several times over the years.

1. Judiciary Act of 1789: Court size 6
2. Judiciary Act of 1801: Court size, 5
3. Repeal Act of 1802: Court size, 6
4. Seventh Circuit Act of 1807: Court size, 7
5. Judiciary Act of 1837: Court size, 9
6. Tenth Circuit Act of 1863: Court size, 10
7. Judicial Circuit Act of 1866: Court size, 7
8. Habeas Corpus Act of 1867: Court size, 8
9. Judiciary Act of 1869: Court size, 9

Most likely, the current court has an odd number of Justices to prevent an evenly split decision. The number may have expanded to nine because the caseload has increased over the years, and not all Justices elect to review petitions for writ of certiorari (requests for a case to be reviewed by the court).

The number was temporarily reduced from ten to seven during Andrew Johnson's term of office, because Congress didn't want him to have an opportunity to appoint new justices. The size of the Court was increased to nine rather than ten (most likely to avoid tie votes) when Ulysses S. Grant was elected.

Congress always had reasons for changing the size of the Court -- whether to allow a sitting President more or less influence, or to adjust the Court's workload, but their purpose isn't always easy to determine.

The last time this was seriously proposed was during the presidency of Franklin Delano Roosevelt. However, Roosevelt's plan (which would have ultimately allowed for up to 15 justices) was so obviously an attempt to pack the court with those whose political ideology agreed with his own that it was a political disaster for him. As it was, 8 of the 9 justices on the Court (including the Chief Justice) were Roosevelt appointees by 1941.