The correct answer is: four.
The Rule of Four manes that if a litigant asks the Supreme Court to review their case, four of the nine Justices have to agree to grant a review called in proper terms a writ of certiorary. A certiorary is an extraordinary writ issued by the appellate court.
It takes four Justices to agree to review a case or the case is otherwise dismissed.
This procedure is not established in the Constitution but it is rather a custom, a working rule designed by the Court as a practise mode to determine if a case is worth of review.