Hi David,
From what I could gather from personal experience and discussions with colleagues, it looks like the ACS 5.0 should go up to 15000 AAA clients.
So following common sense, if your end customer would be deploying less than 10000 AAA clients, they should be covered.
If you'd need official details on more specific numbers, you may need to open a TAC case:
http://tools.cisco.com/ServiceRequestTool/create/
Hope this helps,
Fede