I believe that a game should be a good, long game, with plenty of places to explore. That being said, IMHO, a completed RMXP game would have way more than 50 maps. But then again, I use maps for more than just in game explorations, but also for intro scenes, cut scenes, and the like.
50 maps or less would be hard press to give a truly in depth game that lasts more than 4 hours of game play.