You could equally decide that on average a planetary system has 0.003 planets or 0.000001 planets. Choosing such numbers simply indicates that you think that there are very few habitable planets out there. A system may or may not have a habitable planet and very few have more than one. According to Frank Drake, at the original conference that explored the Drake Equation, the average number of habitable planets in a system was said to be between 1 and 5.
Some people consider Mars to be habitable.
For example, some people consider Mars to be habitable. Evidence suggests that the surface of Mars might once have been habitable and that there may, even now, be subsurface areas that are habitable. Some systems may have none or one, some systems may indeed have two, and others may have three or more. According to Frank Drake, at the original conference that explored the Drake Equation, the average number of habitable planets in a system was said to be between 1 and 5. So with so many candidates in our solar system, maybe there are many habitable worlds here and in other systems.