I don't know if there is an actual limit but if it helps, we have some 30 inbound emails at this point, and I don't recommend it.
The biggest problem I find is that the email boxes are processed in alphabetical order by ID, and each time the scheduler runs it starts again from the top.
This means that if your top addresses get very busy you NEVER get to process the bottom ones and there is no out of the box way of prioritizing which queue is processed first.
So it's not so much a question of how many addresses you have but rather how many emails you have coming in on each address.
We also have a lot of emails coming in that cannot be processed, they appear to be "malformed" based on our Sugar Logs (we are still on Professional v11.0.3 on Site - working to get converted to Enterprise v12), yet our mail server accepted them, and I'm trying to debug what is causing those errors in my log. In other words inbound email can be tricky to manage.
We started our Sugar implementation on Sugar v6 which was a different beast from the current version and are carrying a lot of legacy with us, which I'm working to get away from. With APIs being so flexible today, and the availability of Sugar Portal (support.sugarcrm.com/.../), I would strongly recommend you encourage your users to use the web to communicate on Cases, if you can, and limit your inbound emails to a minimum. It is what I'm encouraging our organization to do.
