- using
ab
- the apache benchmarking tool, or any load testing service in the internet (plenty of them)
- this question is pretty much the same as the first one. yes, there is.
- this depends on the web server configuration, on the memory amount and the free memory amount in the server, on the internet connaction bandwidth.
What is more important - it's not the "number of clients that can connect" that matters, but the number that your setup can handle, and this is not the same thing. If your setup/application is slow - clients will start stacking on each other, filling the server's sockets, queues and etc. You need to have the application fast enough to minimize the time the client is connected to it.