From Why the U.S. Supreme Court has nine justices:
Nine justices make up the U.S. Supreme Court: one chief justice and eight associate justices. But it hasn’t always been this way. For the first 80 years of its existence, the Supreme Court fluctuated in size from as few as five to as many as 10 before settling at the current number in 1869. Here’s how the court ended up with nine justices—and how that could change.