CSA currently supports 10,000 agents managed by a single server with the SQL backend.
Some customers, including Cisco, have more than 10,000 agents reporting back to a single server. However, 10,000 is the number TAC will support you with the current version. Support for more than 10,000 will be available when 4.5 version of CSA is released towards the end of the year.
I couldn't find a document to point you to, but this is indeed the number.
Hope this helps!
peter