The license you need depends on the functionality you want out of ISE. If you want basic AAA, TrustSec, or guest services, a Base License for the number of endpoints supported will suffice. If you want BYOD, Profiling, pxGrid or CA functionality a Plus License is require. If you want MDM or posture support, you will need an Apex license. You can get more information about ISE licensing and ordering here. You will also want to review the Sizing and Scalability guides as well.