From what I understand about the HTTP protocol is that it is stateless. This means (to me) that is, it is only ever serving one connection at a time.
Even if there is 1,000,000 million users trying to access a site, it can only ever be serving one connection at a time.
So when I see a setting in IIS saying "Maximum number of concurrent users" (or similar) it makes me wonder, what does this mean?
In theory, it can be any number until the TCP connections run out.
In reality, it is limited by your hardware and your applications and what the application/users are doing. You need to do stress test for your server.
hope it help.