well, we cannot draw a line on a server, beyond which the server cannot handle. It depends on many many factors. Like what you run, how many websites hosted on the server, how much do you cache your pages and queries and how your MySQL queries are done. Further it greatly depends on the request type.
If you have separated db from your web server and host images on a separate vhost, then your main site will perform much well.
I have read at many places on configuring MaxClients for apache similar to what ianeeshps said above - Find the average memory usage of an apache child, leave some of your RAM for OS and supporting applications, and divide the rest of ram with the memory usage. If I have 8G ram, and my apache is using 100M per process, then my max client is 1024*7 / 100 which is approximately 80 , which makes no sense for me ..
If you use a light weight webserver like lighty or nginx, it'll process more requests than apache do.
So its more a trial and error and better use of available resource, than going for a specific mathematical output.