How many justices must agree for the Supreme Court to hear a case?

For the Supreme Court to agree to hear a case, at least four justices must vote in favor of granting a writ of certiorari. This is known as the "rule of four," which is an important procedural rule in the Supreme Court's operations. The reason this rule exists is to ensure that a minority of justices can still have an opportunity to hear cases that they deem significant, even if the majority does not agree on the necessity of addressing the specifics of that case.

This requirement plays a crucial role in maintaining the Court's function as a check on government and lower court actions, allowing justices to take on cases that might involve important constitutional questions or significant legal precedents. The rule also promotes a diverse range of perspectives within the Court's deliberations. Thus, the requirement of four justices serving in favor of hearing a case is foundational to the Court's ability to fulfill its role effectively.