Tanya,

I've searched for official info on this  numerous times. I don't think it's public, any maybe there isn't even an official limit. From what I understand, each entry is judged case by case by the immigration person at the entry point. They typically ask length of stay, purpose of visit, etc. If a passport shows numerous entries backed up against each other, maybe they will dig deeper into what a person is doing during their visits. 

This is one of those types of questions where I like to remind people that this site, and everything you read on here, is not an official/legal source. Just a forum where people help each other out with common questions 🙂 

If you find anything, please let us know.