You can use as many as you like, experience and practice tends to dictate how to do this. I find from 3 to 5 exposures enough. Bracketing a stop between the "correct" exposure each way.
Google photomatix as an alternative to photoshop. HRD is dead easy once you figure it out, but you do need good PP skills to make the best of it.
I only ever create HDR images from one RAW file as I can't be bothered with tri-pods and stuff.
I just looked this up myself and found the following tutorial. Looks like it will be helpful, I won't find out until I get my DSLR though. This guy's pictures are pretty impressive, although they tend to be surreal enough that there's no doubt they've been digitally enhanced. He mentions using a single RAW file too, although he doesn't go into detail about how the software does whatever it does.