One of the 80's Foundation books (I think Foundation and Earth) claims it's thought as many as half the settled planets in the galaxy were a secret, at least administratively, to the Galactic Empire. (Of course this reflects an Asimov who'd had decades to put second thoughts into things.) (And Asimov's settings usually benefitted from having second thoughts about things.) But if the trade routes are kept vague and out-in-the-boonies enough ... well, there might be coexistent Galactic Empires that barely know one another's existence, with for mild plausibility the fact that with the hyperjump drive there's no reason for anyone to actually know what's between points A and B.