Best practice to implement user licensing in desktop app

In a desktop application, is there a best practice to implement a
check at login time to ensure that the number of active users has not
exceeded a limit?

In our application, we intend to sell licenses for the software based
on the number of concurrent users that need to be supported. What I'd
like to do is check at login time how many active users still have the
application running. If there was a license available, I'd let the
user start the application...otherwise I'd display a messagebox and
abort the application. What's the best way to do this?

This is a .NET desktop app with a SQL Server/MSDE database as the
backend. Has anyone dealt with designing a function to check how many
users were still actively running a desktop application?

Thanks for any ideas!

--steve