Limiting simultanious EXE execution to a certain number

I have an exe that is run from a hidden share from the logon script. This way, all the computers who sign onto the domain have to run this program.
We sell our software based on the number of PCs that will be using the software.

A problem exists though: How can I tell how many computers actually ran/accessed the exe?

If we sell to a company that has 100 computers, nothing is stopping them from adding 200 more computers after installation.

Is there a way to limit the number of connections to a file?

You will need to maintain a file or database table that records either the number of current users or an actual list of those users. You will need to have a way to decrease the number or remove a user from the list when they shut down their PC or log off. You will also need an administrator's utility to reset things for those times when a PC crashes.

That's a good idea but I was hoping for a simpler solution. It is possible for windows to display open file shares and who has them open. Maybe I could do that thru api and look for my exe name. Anyone have ideas how I could do that?
Thanks

Provided you can do it, an API method that checked shares would be much simpler. You may need a copy of Dan Appleman's Windows API book to figure out how (or even if) it can be done though.

The way I see things, if any Windows program or applet, then it must be done thru "normal" api means. I don't think most people think about it, but everything that is part of Windows was made using "normal" methods. Taskbar has normal components, is always on top and makes itself a toolbar. Add Remove control panel applet enumerates registry entries. So there also must be a way to check any shares, just as Windows does.