To answer your questions of this post :
- by default (without additional licenses) only two remote sessions (terminal services/rdp) are allowed (for maintenancfe purpose of the server.
- you need to enable TS licensing on your server where you have two choices : license per device or license per user. So what you need to buy is TS CAL (Terminal Server Client Access License) for your operating system either by user, or by device. As I suppose each roaming user have their own device, I would suggest to buy a "by user" licenses. You can get a quote at any Microsoft reseller.
- yes each user needs a license. Those licenses are installed on the licensing server though. Nothing to install on each client !
- you need a license per every user (concurrent users has no meaning in Microsoft context)
- yes your license server can be on the same machine. Usually it's easier to install this feature on a domain controller as it will then be available to all TS servers without additional configurations.
- about hardware, performances will tell you (number of concurrent users). Key factor here is RAM and disk access.
About the originating question, here is a good reading :
http://shell-shocked.org/article.php?id=284Pierre