How many users/domains can I install on my server?
We get this question quite a bit, and our answer is always "it depends".
The number will depend on numerous variables, usually impossible to accurately nail down until you actually have the users on you box.
We can say that on average, most people have approximately 400-600 users/domains on their system. However, there are many variables that can allow you to add more or fewer domains to your system.
If each domain is static html, there are not any databases, and they have very low traffic, we've seen servers with several thousands of domains. On the other end of the spectrum, some domains are so large, cpu intensive, and have such high traffic that 1 server alone cannot handle the capacity of the domain (has to be broken up into parts, databases on one box, php on another, etc).
So if you need a number to start with, go with 400 as a starting point, just to be conservative, and your load will most likely be fine (room to grow). However, the true number can be higher or lower depending on your clients, their traffic and their scripts. This number cannot be accurately determined until you can acutally monitor the load based on what's there.
Also note that a website can start out small, but things can change as a website grows, so it's always a good idea to leave breathing room in your server for sites to grow. They very rarely ever go down in resources.