I haven't seen anything about a limit in the rules or guidelines. I assume if there is a limit and you get close to it, you should get some kind of warning. I know that is how some of my email accounts work. I have received messages that I was exceeng limits on my emails, and the oldest ones would get deleted, but that was thousands of emails before that happened. As for TER PMs, I have over 500 in my folders. and no warnings.