At our current level of technology detection methods, it's possible, plus we've only been at it for a few decades. Most of the planets we've found so far are gas giants and super earths, we haven't really found earth sized and smaller objects as we can't detect them yet, although I think theres a couple of candidates out there.
Really though, we've only just begun surveying our corner of the galaxy, there will be plenty for future explorers and planet hunters.
Edit: Like Darvince said, we've only covered a tiny fraction of planets, and some systems we are still finding them.
Although, if you want to count rogue planets (anything shot out of the system and are wandering), it's estimated that there are lots of them out there.