I think this has been answered many times.... in short, User numbers is irrelevant and not a measure that can be used for the question which I think you are really asking, which is "can AwareIM scale". The user numbers depends on what your app does, what the users do, and how you configure and tune your environment. As Vlad has said before it is like asking, how many users can PHP support, there is no sensible answer to that question.
If your question is about scaling, here are some of our learnings from the last 6 years of pretty strong growth...
a. growth is never as fast as you expect or wish for, so you have time to learn (sorry to be realistic)
b. hardware scaling these days is cheap, so worst case you can usually throw more hardware at the problem while you learn to do it properly (we scaled up instances on AWS while growing and have since scaled some back now we have a better idea what we are doing)
c. as you scale you will need more knowledge and most likely you don't have it, so you will need to buy it. The best thing we did about 2 years ago was get a tomcat/java/linux guy to optimise our prod servers, he's a regular contributor to our team.
d. optimise your AwareIM rules when you have finished building
e. use a cloud based offering where you can take advantage of scaling cheaply and easily IF you think you will have large growth
