Why do you want to test it on localhost if this doesnt reflect your real server envirement?
You can do these kind of test (but not on localhost) with a website called https://www.blitz.io/
It shows you exactly the response times for your server, how many users it can handle till producing errors, etc.
But you need to do this with a real server, not on localhost. If you test it on localhost the result won’t say anything about the real performance on a real webserver.