Without loosing detail competed to your 12 megapixels camera. If you have a 12 megapixels camera and a good 200 mm lens a 50 megapixels camera will allow you to crop like it was a 400mm lens without optical disadvantages
If you multiply the megapixels by 4 you double the resolution that means a 50 megapixels camera will allow you to crop the picture to the level of a 2x teleconverter