In my own ATU I surmise that there are plenty of sapient species, but only 4 besides humanity are technologically advanced and in space, although the relics of dead civilizations that made it spaceward can be found all over the place (about 30 recorded). Part of the overarching mystery of the campaign is exactly why, when new species make it in to space, they seem to go extinct within one to two thousand years at most....
That said, I tend to operate on the premise that "where life can evolve, it absolutely will evolve." Intelligence, on the other hand, is a whole different affair, but since it's an RPG, I like to liberally sprinkle sapient species around for fun and adventure....
EDIT: I also use the premise of "FTL travel is very, very difficult to achieve" as the limiting factor in why contact with other species is rare, until FTL becomes possible.