The United States Supreme Court has nine justices—one chief justice and eight associate justices. It was created by Article III of the U.S. Constitution that organized the judicial power of the Federal Government into “one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish.” However, Article III did not provide for the composition of any of the courts in the newly established Federal Court system. That responsibility was left entirely to Congress. So how did we arrive at the number of nine justices?
Judiciary Act of 1789: The Supreme Court was initially set up with a chief justice and five associate justices by the Judiciary Act of 1789, enacted by the newly elected first U.S. Congress in its first session. President George Washington then nominated six justices (one chief justice and five associate justices) to fill the court, and also had to nominate a replacement for one justice who declined to serve. Even the position of Chief Justice was not specifically created by the Constitution, but was alluded to in Article I, Section 3 as: “When the President of the United States is tried, the Chief Justice shall preside:” as the Constitution addressed the impeachment process. Although the original number was set to six, a quorum could be reached with five justices. The extra justice was appointed to help achieve the quorum since all Supreme Court justices had to also serve on the newly created circuit court bench as well as the Supreme Court. The circuits were initially set up into three regions, and from time to time a Supreme Court justice might be absent from a session while serving on the bench of one of the circuit courts. The six justices could have deadlocked into a three/three split decision. However, at the time this was not considered to be a problem as all the original justices were in the same political party—the Federalists.
The Adams/Jefferson Controversy: The Supreme Court has been shrouded in controversy several significant times since its creation. The first time was in 1800 when the political composition of the Congress shifted parties, and President John Adams (a Federalist) was defeated by Thomas Jefferson (a Democratic-Republican). To reduce the chance that Jefferson would have the opportunity to nominate a Supreme Court justice, Adams and the Federalist party passed the Judiciary Act of 1801 decreasing the number of justices from six to five. Further, Adams rushed through the nomination of another Federalist to replace one who resigned owing to declining health. This was all made possible because the lame duck Congress remained in session past election day all the way to the month of March, when the new president took office. However, when the new Congress was seated, they repealed the Judiciary Act of 1801, thereby increasing the number of justices back to six.
The Dred Scott Case: Before the outbreak of the Civil War, the number of justices was increased to nine, to enable the justices to serve additional circuit courts in the new western states. That number was increased to ten during the Lincoln Administration in a rebuke of the Dred Scott Case that contributed to the Civil War. After the end of the Civil War and Lincoln’s Assassination, the Congress clashed with the new president Andrew Johnson, and voted to reduce the number of justices to seven thereby restricting Johnson’s chances to nominate a justice. In 1868, Ulysses Grant replaced Andrew Johnson as president, and the Congress increased the number of justices back to nine where it stands today.
The Great Depression and New Deal Legislation: During the Great Depression of the 1930’s, large portions of President Franklin Roosevelt’s New Deal Legislation were modified or blocked by Supreme Court rulings. Roosevelt then established a plan to increase the number of justices to as many as fifteen to bolster support for further New Deal Legislation. The proposal would require any justice that reached age 70 and who had at least ten years of service, to retire; and if he refused, Roosevelt could add another justice to the bench. At the time, there were six justices over age 70, and Roosevelt stood to add another six to the nine already serving making a total of fifteen. However, the proposal was met with widespread opposition as a threat to the Supreme Court’s independence, and was never taken up by Congress.
Conclusions: We’ve notice political chicanery take place in recent times over the nomination and appointment of Supreme Court justices. This might seem to be outrageous; however, we must understand that it has been taking place in the government from the beginning. The single most important attribute of the Supreme Court justices is to remain independent, as referees should be in a football game.
Sources: Why do 9 Justices Serve on the Supreme Court? By Dave Roos,
History.com
Wikipedia, Supreme Court of the United States.
National Archives, The Constitution of the United States: A Transcription.

Politicians are always trying to bypass the rules and stack the deck in favor of their party. The only real check to their power is an independent judicial system and a strong and independent press.
Thanks David