You need both the endpoint licenses (base/plus/apex) and one ISE license for each virtual appliance you install. You probably want to install at least two for availability.
More information is found in the ISE ordering guide:
http://www.cisco.com/c/dam/en/us/products/collateral/security/identity-services-engine/guide_c07-656177.pdf
--
If you found this post helpful, please give it Kudos. If my answer solves your problem, please click Accept as Solution so others can benefit from it.