The article I referred to doesn't directly say that, but hints at it. Here's part of it"The fact that a decision was affirmed by an equally divided Supreme Court is not an independent ground for granting a rehearing. However, rehearings have been granted in situations where the Supreme Court had affirmed judgments by equally divided courts, but soon thereafter a new justice was appointed, who could break the tie on rehearing."
The U.S. Supreme Court has nine members. They used to be called "The Nine Old Men" until Justice O'Connor joined the court. I guess now they are the "Seven Old Men and Two Ladies".ptlaw
There may not be a rule. A tie means that the court cannot reach a decision (no majority). The lower court is affirmed by default.On the other hand, there may be a SC rule somewhere. I haven't found them, yet.As to the number of justices, FDR tried to 'pack' the court by adding a number of justices. I think he tried to go up to 12.