Why does the supreme court have 9 judges

They have 9 justices they each serve a different purpose. they hear cases and controversies arising under the constitution or US law and controversies that involve the United States as a party of entities and parties of different states that are appealed from federal courts or state courts there hasn’t always been 9 justices on the court the US Constitution establishes Supreme Court but they left the Congress to decide how many justices should make up the court the judiciary act of 1789 said the number at six but that later changed