Print Distributor is licensed per machine it is installed on, no matter how many clients connect to the virtual printers.
There are two editions, Workstation and Server, the only distinction between the two is which edition of Windows they run on:
Print Distributor works on Windows 7, 8, 8.1 and 10. The Workstation license key will not work on a Server edition even if you are running it as a client.
Note you can still share virtual printers from the Workstation edition up to the Windows client limit of 10 users.
Print Distributor Server works on Windows Server 2008, 2012 and 2016. The Server license code will not work on a client machine, if you need a client license for support then you will need to purchase this in addtion to the Server license. You can share the virtual printers to as many users as your Windows license supports.
When you purchase Print Distributor you will receive a license code by email, it will look something like this:
Use the Help->Enter Registration Key menu in Print Distributor to apply this license:
Note: Your computer does not need to be connected to the internet to apply a license key.