The same method is used in most mobile games.
You've got X amount of chapters, usually found in a row which can be selected via touch. Once a user selects a chapter, a grid of levels opens up, usually something like 3x5 or so. 50 levels would be a stretch to fit in one screen without it looking too cluttered.
Take a look at angry birds, stupid zombies, etc. They can give you an idea of how to layout a level select scene.