You only need one sensor station, you can get the rest of your range with sensor equipment. Each sensor equipment block has about 6km range. The sensor station has 6km range as well.
100km-200km range will allow you to scan any planet and its moons while orbiting the planet.
2-3 million km range will allow you to scan every planet and moon in the system while orbiting the star. (Or standing next to a wormhole, as they are pretty close to the star)