Here's the simple explanation as I understand it (and how we use it at our clients):
* Per Device - Licensing is done based on specific machines connecting to Terminal Services. If you have 1000 users, but only 500 clients (like a dispatch center), then this is ideal - you pay for 100 clients. The downside is that the connections are machine-specific - you will need to pay for client #101, and be sure that Terminal Services understands that old clients (client #1 is replaced, for example) are removed. I've never used this model, as it can get very messy.
* Per User - Licensing is done based on CONCURRENT users. If you have 500 users that may bounce between multiple (and frequently unreleated) clients, this is a better fit, and far easier to manage. Terminal Services will monitor connections made to the Licensing server, and will release users as they log off. The catch is that users connecting to the TS via RDP need to be sure to log off - I've seen licensing servers get confused, and hang on to unused users, long after the user "logged off" (translated: closed the window). I've used this model more frequently.
In your case, it sounds like you have a rotating staff over the course of the year. You only need to get licenses for the number of CONCURRENT users you will have. Microsoft does offer discounts on retail multi-user packs (10-pk, 25-pk, etc), or you can contact your Microsoft reseller to negotiate a steeper discount on more users.
Hope that helps....
Mike Molenda
RSA Corp - Houston, TX
Technical Assistance Center