The specification N40 is merely an indication of the magnetisation quality. The strength of a magnet depends on its shape and size as well as the respective magnetisation quality.
For magnets of the same size and shape, a higher magnetisation quality (indicated with NXX) means a higher adhesive force.
We mainly use N40 for our sphere magnets
You can find other magnets with a very specific magnetisation here: Magnets N35, Magnets N42, Magnets N45, Magnets N48, Magnets N52