You will need a separate license for each installation of NPrinting Designer 16.x.
NPrinting Designer & Server serve different purposes. Designer is used to create NPrinting Projects (NSQ files) that contain most NPrinting entities like Connections, Recipients, Report definitions, Tasks, Jobs, Schedules etc. Server can then be used to run these scheduled Jobs. Take a look at these threads:
TH - Daniel.
I think we use such scenario. We have citrix with two virtual machines with np16 designers for multiple users.
If am assigned to either of the machines I use the designer registered on that machine - not my account.
There are 5 users and I never had any problems with license usage but I am not sure if we ever used it concurrently.