And, well, in that sort of movies, the credibility often leaves place to well known dramatical effects: even surounded, the heroes win. Many "good guys" die, but the real heroes are alive at the end... quite simple... (who said too simple?)
But finally, it's an interesting point. Maybe it would have been better to see more Rebel ships. Many of them have been added for the SE, but having hundred ships instead of few could have changed too much that part of the movie, I guess.