Hi Kader.
Performance will stay linear for end users up into the tens-of-thousands of Application Groups. I've worked on systems in the 7500+ range, and aside from the admin client being a little pokey (it has to download those definitions when you open up the windows) it will stay fast, regardless of how many documents you cram into it, providing you configure everything else correctly (segment sizes, expiration types, etc.)
-JD.