Let me explain our licensing. USB for Remote Desktop consists of two components: Server and Workstation. Workstation is a free software, so you need to pay only for Server(s). 1 installed Server = 1 license. However 1 license can be of different types. Each license key enables connecting of the specified number of USB devices.
1 USB device $ 199 5 USB devices $ 699 10 USB devices $ 999 20 USB devices $ 1599 50 USB devices $ 3799
For example, if you have 1 machine where 1 USB for Remote Desktop Server is installed and you need 5 devices to be connected to the Server simultaneously - then you need 1 license 5 USB devices. And it doesn't matter how many clients are connected to the server. It can be 1 client with 5 devices or 5 clients each with 1 device.
For instance: Let's say I purchase the 1 USB device because I am interested in remote scanning. The workstation has four USB devices connected to it. One is a printer which is already supported in TS, one is a flash drive which I'm not interested in accessing when connected to the TS, one is an external backup drive which I'm not interested in accessing when connected to the TS, and then the scanner which I do need to use when connected to the TS.
Do I get to pick which USB device the license counts towards?
Well, the only thing that counts is the number of USB devices connected to the "USB for RDP" Server.
In your case you will redirect and connect only 1 device (scanner). So you will need 1 license for 1 device.
Actually you can redirect as many devices as you want. But it will depend on the type of license how many devices you will be able to connect to your Terminal Server. The simultaneous connections between the client and the server is the thing that counts