I have spent time trying to optimize my website but have never got a chance to test maximum users it can take. Have been doing some reading and found out the best scenario would be to use cloud web load testing. The only catch is that its ridicously expensive. Is there any service that can be used to test this that is free and can simulate real browser users.
There are benchmarking tools for Apache2 available, take a look over here
It shows you how you can try to emulate an amount of requests. There are some requirements to make it as reliable as possible, but I guess it should do the trick. There is also a site http://loadimpact.com but I do not know it's reliability.