Hi R0J0hound
Top wall is a good solution to prevent the blocks falling out. In this way and changing the the physics engine to Box2D asm.js, now I have 32 squares on the screen.
I enabled the bullet option in the physics properties and the movement is smoother. I like it, but instead of 32 now always appears 28 (remembering that I decreased the thickness of the walls and added one on top).
Edit: Bullet option enabled is better for mobile?