Interestingly enough, we've never actually seen a planetary invasion in Star Trek, so we don't really know how many people it takes. Wars a thousand years ago could be won or lost by a couple of dozen horsemen, even if tens of thousands of fighting men participated. Global wars in the Trek 24th century might be decided by a thousand people with hand phasers. Plus their starships in orbit, of course.
We sort of did in Voyager, when Seven was forced to go back to the Borg when the Queen threatened to destroy Voyager if she didn't.
We saw hordes of Borg cubes enter orbit over some planet, destroying about everything and assimilating natives.