The number of exposures needed depsnds on the dynamic range of the scene in front of you. As Gary says the bare minimum is three.. but in my opinion any scene where you can capture the entire dynamic range with three shots +/-2 isn't worth the bother of HDR. It's a technique that really comes into it's own when you've got a scene that needs six or more exposures to capture fully.
Yes possibly to perform it correctly on a very static scene, but to do it with a single shot I've found under exposing by .7 and then adjusting the exposure in your raw processing does the trck.