13 screws: they're all the same size, so you can avoid taking note of their position.

Note that the screws in the middle are at the bottom of a 2 cm deep hole. You will need a relatively thin and long screwdriver, as the one in the picture.

There is a smaller screw holding the battery cover. You can just ignore it.

In the lower part of the picture you see a different screw that I added to hold in place the shim, as explained in the next step. (You can avoid using this screw by making a more precise shim or just using some glue).