The NTA license are based on interfaces. Suppose if there are 500 interfaces that you wish to monitor then you need to get 500 licenses. If you want to monitor an sub-interface you need to have a license for that as well.
Example: if you want to monitor eth0/0 you need 1 license, and suppose this interface has an sub-interface eth0/0.1 then you will need an additional license to monitor eth0/0.1
You must get the same license that you have for Orion. So if you have Orion SLX then you must get NTA SLX.