Can you explain a little more about the software of which you speak.
You mention it runs in the browser. Generally the simplest and most reliable thing to do is to protect from the server end of such a system. Do you have .NET code under your control on the server side that would be in a position to know what the current number of instances are available? How do you deliver your software? Who installs it and what is the nature of the hosting arrangements?
Given a bit more detail covering the above aspects, it should be possible to work out something suitable.
Regards,
--Ruben