Thanks for the response.  It is currently a networked windows app (database program that runs on top of SQL Server).

The new version will have two different types of users, one that will be web based with reduced functionality, and one that is still a windows app.  (there "goal" in the long run is for it to all be a web based app, but that is down the road, and I still personally believe that there are some things that will never be as easy over a web browser than they are through a truewindows application)

I am not sure how the current version of the software keeps track of concurrent users (but it clearly lets us know when we hit the number)