NO! There is no reason for 9 Supreme Court Justices
The US Supreme Court is established by Article III of the US Constitution which reads as follows:
Section 1. The judicial power of the United States, shall be vested in one Supreme Court, and in such inferior courts as the Congress may from time to time ordain and establish. The judges, both of the supreme and inferior courts, shall hold their offices during good behaviour, and shall, at stated times, receive for their services, a compensation, which shall not be diminished during their continuance in office.
Section 2. The judicial power shall extend to all cases, in law and equity, arising under this Constitution, the laws of the United States, and treaties made, or which shall be made, under their authority;–to all cases affecting ambassadors, other public ministers and consuls;–to all cases of admiralty and maritime jurisdiction;–to controversies to which the United States shall be a party;–to controversies between two or more states;–between a state and citizens of another state;–between citizens of different states;–between citizens of the same state claiming lands under grants of different states, and between a state, or the citizens thereof, and foreign states, citizens or subjects.
In all cases affecting ambassadors, other public ministers and consuls, and those in which a state shall be party, the Supreme Court shall have original jurisdiction. In all the other cases before mentioned, the Supreme Court shall have appellate jurisdiction, both as to law and fact, with such exceptions, and under such regulations as the Congress shall make.
The trial of all crimes, except in cases of impeachment, shall be by jury; and such trial shall be held in the state where the said crimes shall have been committed; but when not committed within any state, the trial shall be at such place or places as the Congress may by law have directed.
Section 3. Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open court.
The Congress shall have power to declare the punishment of treason, but no attainder of treason shall work corruption of blood, or forfeiture except during the life of the person attainted.
NOTE that nowhere in the Constitution is the number of justices other than the use of the plural “Judges.” As long as there is more than one judge, it’s a proper court according to the Constitution.
When the Supreme Court first convened in Philadelphia (Old City Hall) in 1789, there were five Associate Justices in addition to the Chief Justice (The Constitution doesn’t specifically create the office of chief justice of the Supreme Court, other than to say that that the chief justice of the Supreme Court is to preside over the Senate trial of a president who is up on impeachment charges.) for a total of six. From 1790 to 1869, number of justices has varied from 5 and 10. This legislation was reversed in 1869 after Ulysses Grant was elected president and the number of Justices has remained nine ever since but there is no reason for this to be true. Other than Congress has not sought to change it. Congress wasn’t the only branch of government to attempt to alter size of the courts as a political expediency. President Franklin D. Roosevelt proposed a reorganization bill to Congress that would allow the president to appoint a new justice for each one who was at least 70 years old. Congress did not oblige, as this was seen as a court-packing scheme. Roosevelt continued to fight with the hyper conservative Supreme Court as he pushed through his New Deal.
So, adding Justices is a bad idea, right? Well, not if you listen to judiciary scholars. The current court has gradually become less of an independent third branch of government and more of an extension of the two-party system. Judges have routinely voted along party lines and the number of split decisions does not create great confidence in the independence of the judiciary. Whether the courts need reform is largely a question these days of which party is in or out of power and like Roosevelt, various Presidents (not just Trump) have sought to skew the ideology of the courts to the right or to the left. Furthermore, how we redefine the courts could prove to be as difficult as whether or why so don’t count on it ever happening.
Options to reform include simply expanding the Court is adding two, four, or six new justices to the Court. This expansion would serve to offset the tarnished confirmations of the most recent two Supreme Court nominees but could also unleash a spiral of retaliatory moves by whichever party is in power. The second option is to reconstitute the Supreme Court in the image of a federal court of appeals. This course of action would increase the number of justices to fifteen or a similar number. Panels of justices would be drawn from this larger group, with an option of en banc review. This plan would not only dislodge the Court’s current reactionary majority, but the panel format also would allow a greater number of cases to be heard something that is in the best interest of all parties and all people. A third option is a Supreme Court Lottery. In this proposal all federal appellate court judges, roughly 180 in total, would become associate justices on the Supreme Court. Panels of nine justices would be randomly selected from this pool like juries. Importantly, decisions on whether to grant certiorari on a given case would be made by panel members who would not know the ideological makeup of the panel that would hear the case. Thus, this plan would frustrate partisan maneuvering. Finally, there are proposals to build a “Balanced Bench” for the Supreme Court. This proposal aims to counteract the effects of partisanship on the Court by explicitly recognizing and institutionalizing partisanship presence. The Court would have ten justices, with five seats allocated to each of the two major parties. Those ten justices would select sets of five additional justices at a time to serve a future, non-renewable one-year term. Of course, the problem with this proposal is it entrenches the train-wreck politics of the two-party system and prevents a third party from ever getting a seat on the Supreme Court.
The real solution to all these problems is the complete and utter dissolution of political parties but since these abominable institutions (both parties) are focused only on their retention of power, that is an impossible dream.