The number of images you use will depend on the image itself. Honestly, I feel like more than 5 creates more problems then it solves. In most cases, 3 will usually suffice depending on the range of EV values. Choose your over-exposure and under exposure ranges carefully, however, or you will either not get the result that you want, or you may get some weird banding in low and midrange values. It is a common misconception that your brackets always need to be "symmetrical" around the "average" one. Your brackets should reflect the areas where you want to retain texture in the low, mid and high values. And you should always have the camera on a tripod or firmly braced so all of your images are in the same "register".