Maximum users supported by the ACS internal database
—There is no theoretical limit to the number of users the ACS internal database can support. We have successfully tested ACS with databases in excess of 100,000 users. The practical limit for a single ACS authenticating against all its databases, internal and external, is 300,000 to 500,000 users. This number increases significantly if the authentication load is spread across a number of replicated ACS instances.