Not sure if its mandatory to purchase the CON-ECMU-LCT50A with the adder licence. Recommend contacting Cisco licensing: http://www.cisco.com/go/license or i believe their email address is licensing@cisco.com
Reference the N+1:
https://www.cisco.com/c/en/us/td/docs/wireless/technology/hi_avail/N1_High_Availability_Deployment_Guide/Licensing.html
A secondary controller running a minimum AP count license and configured as an HA- SKU controller provides the maximum AP capacity as supported by the hardware. For instance, a 5508 WLC running a 50 AP count license and configured as an HA-SKU secondary controller provides support for 500 APs.
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