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.