How to judge the breakpoint or bottlenecks of the tested server using Loadcomplete.

I am unable to judge that from which point the load is not handled by the website/server,as reports shows in terms of response time and passed request,but how can i know that,after this point server/website will be crashed.

Basiclly what amount of load the site can handle in terms of approximetely maths figure of virtual users.

Thanks in advance.