How many times can a center jumper tap a jump ball before it becomes a violation?

Tapping the ball more than two times before it hits the floor,non-jumper,basket or backboard is a violation.Rule 6-4-7(d).

Of course, JR is correct. However, since I have seen a second tap incorrectly called a violation numerous times, I'm responding to this to emphasize that the rule says "more than twice." This means that two touches is allowed, but not three.