There is a Microsoft document explaining the licencing terms here:

Each thin client would require a Terminal Services Client Access Licence (CAL) and the server itself needs enough terminal services licences for 10 users.

But at least you're now aware that there are many possible choices.For the 'plain' Windows approach, you would be installing a core server, such as Windows Server 2008 with Terminal Services running. And I do think we've just about exhausted terminal-services-as-a-migration-aid for now. Thanks to reader Dave Dabinett for making me aware of NetLeverage. While you're checking out ThinPoint, you might also take a look at NetLeverage's NetPoint appliance - a 6-server-in-1-box product that drops into your network and provides core network services such as file and print, mail and Web, fax, and Internet connectivity along with heavy duty perimeter security, intrusion detection/prevention, anti-virus, and anti-spam protection.

And even once you've completed your desktop migration, ThinPoint can remain as your remote access server, dishing up applications on demand for your users, no matter how far away they may be. Unlike Windows Terminal Server, ThinPoint is licensed by concurrent user, thereby keeping your costs down. In particular, note that it is eDirectory-enabled, so authentication is no problem. See the spec sheet PDF for all the technical specs. It's simple, easy - and very cost effective. No need for dual boot, special client software or any other modifications. Instead, those users who need it simply use their Linux-based Web browser to go to the ThinPoint appliance and run Excel when they need to access the spreadsheet. Here's a typical scenario - you migrate your users to Novell Linux Desktop running OpenOffice, but there's a pesky spreadsheet that uses Excel macros extensively that might need to be rewritten. Headquartered just outside Sydney, Australia, NetLeverage offers ThinPoint and, which it calls 'affordable secure remote access.' Well, what's not to like about that? Delivered as a plug-and-play appliance, ThinPoint hosts up to 120 users at a time allowing Web browser-based access to your standard office productivity tools - no need to buy special versions or licenses. There's one more name to add to the suppliers of thin client computing solutions to help with your transition from Windows to Novell Linux desktops, and it comes from down under.

Thin client windows terminal services