Consider the rook pawns (the outer ones). If they capture towards the middle all the way, that is the furthest right and left that they can go. Some of these squares are in common. All of the other pawns can get to those common squares. The common squares are shown with “C” in the diagram below. There are 12 of them.
\C C C C C C/ \C C C C/ \C C/ / \ / \ / \ RP P P P P P P RP