On average, how many stars have planets orbiting them?
An impossible question to answer.
It is only within the last 20 years that astronomers have discovered planets outside our solar system. They have discovered about 715 planets revolving around 305 stars which means slight more than 2 planets per star.
Given that our medium size star has 8 full size planets revolving around it, it is not unreasonable to believe that the planets we have discovered have another 6 or 7 planets with them revolving around that one star.