You can use Flood to figure out this type of information. If you're on a shared or reseller hosting account you can basically forget about using this to test your host since it would certainly violate any TOS they may have with regards to CPU usage ect.
Well, the answer is not simple as the mathematical number. Its actually depends upon your type of contents i.e php, static, mysql) and how much average file size do you have and how much hits on average would one page be.
if u have dedicated server then yo can find it out by frequently watcing at the server, it depends on the server capacity including RAm. processor and configurations on web
likely for apcahce web server
you can use top -c command and SHIFT+M TO see the mb of ram utilised for a single process
then you can divide with that to total Mb size of RAm ( 8GB RAM= 8*1024)
you can see the number of httpd process by the command pgrep httpd|wc -l
it depends upon your webserver configuration, it doesnt means that not higher than this number is possible. just to optimize your webserver...
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.