Are you saving HD video files? Then you will probably need a data store connected to your web server.
Are you just allowing people to post text, small images and such? In that case, 1 server could easily cut it.
With proper software configuration, and a decent hardware config, you can go quite a long way with 1 machine.
Can you provide any details? Maybe someone can help out with your specific question, since the general question you've asked is quite difficult to give a proper answer to.