Basically this is how it works:

Request for re-exam

Request is published

Substantial new question (if yes)- sent to examiner

(if no)- request denied

If no petition is filed request is terminated

If there is a petition, then the director reviews and there is always a possibility that a substantial new question if found at this point.

There is no limit to the number of re-exams that may be requested. It is all about how important the patent is.