The size of the vent line through the roof (and internally as well) is covered by local codes. Essentially, it is figured by fixture units...add up the values for the things it is venting, and that determines the size of the vent required. In some places, they require the part going through the roof to be larger than the parts down lower to prevent the water vapor from condensing, freezing, and closing the vent off. If you made the one roof penetration large enough, you could use just one, but location in the house may make that impractical. To prevent pooling of rain or condensation, the vent needs to be sloped, just like a drain. This can make long runs impractical as well.