I still don’t understand how you would like to handle this. When people are paying for a service they expect the service to work. What if they cannot log in at all as there are always 2 people online? Are you expecting them to sit there and click on the login until they can finally log in? I don’t think that business model would work and it would cause more problems than benefits.
You can turn off the credit card module to avoid that more people buy the access than what your system is capable of handle. That way you can avoid to have angry customers.
Is there any way you can implement a way to switch off payments when a certain amount of people have subscribed? If this happens, a different page could be displayed telling the would be subscriber the system is at full capacity and to check again tomorrow or a message telling them more bandwidth is being added to the system. The paid customers can still log in and are not affected. I think this would be a unique feature that would put you a cut above the rest 🙂