Some do not have long enough life spans for life to evolve. The fraction of stars that have planets. We believe that planets are unlikely in binary or multiple star systems. However, we've recently found planets around many nearby stars, so this fraction could be as high as half. I'll be optimistic and say 0.5 for this fraction.
Current theories suggest that habitable planets are common.
Unfortunately, most extrasolar planets we've found are huge, and many have surprisingly small or irregular orbits. They might prevent formation of earthlike planets. You could predict as many as two or three planets. I'm afraid I'll have to guess lower. I'll say one planet in half the systems, or 0.5.