how can i get to know about my web site maximum capacity for the visitors at the same time? -kind of stress test for unexpected situations-
You are correct when you think in terms of stress test. You need to be able to reproduce the amount of users you are expecting in order to know precisely how many concurrently users your application will be able to handle.
You start with a low number of users and then you can increase it until you reach a point where your app stops answering in a acceptable amount of time.
I'm afraid there is no simple answer to this, but the simplest way to do this that I can think of is write a simple script that will make GET/POST requests (maybe even using wget) and run it on a farm on Amazon EC2 or something like that so you can truly reach the max capacity of your infrastructure.