Hi,
Its for a single installation only.
The following from the ACS FAQ will answer your question:
Q. What is the licensing for Cisco Secure ACS Version 3.2?
A. The Cisco Secure ACS product is licensed per server, with unlimited ports, users, and network access servers. For available part numbers and descriptions, refer to the Cisco Secure ACS 3.2 product bulletin at
http://www.cisco.com/go/acs.
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Note: Minor upgrades (for example from Cisco Secure ACS 3.1 to Cisco Secure ACS 3.2) require either a purchase of the Cisco Secure ACS 3.2 upgrade (CSACS-3.2-WINUP-K9), or a current Cisco Secure ACS support and maintenance license (CON-SAS-CSACS-3.x). A customer with a license for Cisco Secure ACS 2.x is not entitled to 3.2 under maintenance and must purchase CSACS-3.2-WINUP-K9.
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Q. Can I purchase or license a backup server?
A. No, a separate Cisco Secure ACS server license must be purchased for backup and recovery purposes. Cisco Secure ACS servers can be administered to run as a recovery or failover server configuration. Because Cisco Secure ACS is a central control service in your network, Cisco highly recommends that you configure a backup server for failover and recovery.
Rgds
Paddy
