How many worker processes are assigned to an application pool

One worker process is enough in my opinion. Consider that requests are queued, not rejected, when another request is being served and no more threads are available in the process (I'm assuming that the process is multi-threaded).

I know this is an old question, but it came up under related questions so I thought I'd chime in for those who might come across this at a later date. Scott Forsyth recommends against running web gardens because of the many challenges they raise. He has a video on the subject which he intros with:

This week’s lesson discusses why it’s rarely worth using Web Gardens and why using them is often detrimental to your server’s performance.

The biggest caveat is the implications for session state, but there are other challenges as well.