Cisco Multi WLC Redundancy

Firstly there is no license needed for WLC high availability except for the AP count license, the feature is inherently available on the box as long as you have enough AP licenses.You can use the WLC for redundancy, please check the links below for the configuration example, best practices and FAQ.

WLAN Controller Failover for Lightweight Access Points Configuration Example
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_configuration_example09186a008064a294.shtml#c4

Wireless LAN Controller (WLC) Configuration Best Practices
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a0080810880.shtml

Wireless LAN Controller (WLC) Design and Features FAQ
http://www.cisco.com/en/US/products/ps6366/products_qanda_item09186a00808b4c61.shtml

WLCs need to have enough AP licenses to cater for all the access points should a single controller fail. There is no option currently of sharing AP licenses from one WLC to another WLC. An example of this is if you have an environment with 1000 APs and 3 WLC you would have to license each for 500 APs. This is to cater for the event of a single WLC failure there will still be 1000 AP licenses between the remaining operational WLC. If you have a particularly mission critical wireless environment you might consider 4 WLC each licensed with 500 APs. This way you can loose 2 WLC without incurring any Wireless AP outage.