None of Microsoft’s licensing is based on concurrent users. Each user that has accesses a Remote Desktop Server or Terminal Server needs a licence. This is independent of the number of simultaneous sessions being used on the server. Licences can be reassigned to other users / devices, but only in the scenarios described below (from http://www.microsoft.com/windowsserver2008/tr/tr/licensing-terminal.aspx):
Remote Desktop Services Client Access Licensing Mode
Remote Desktop Services CALs are available in Per User/Per Device mode only.
In Per User or Per Device mode, a separate RDS CAL is required for each user or device that accesses or uses the server software on any server. You may reassign a RDS CAL from one device to another device, or from one user to another user, provided the reassignment is made either (a) permanently away from the one device or user or (b) temporarily to accommodate the use of the RDS CAL either by a loaner device, while a permanent device is out of service, or by a temporary worker, while a regular employee is absent.