The second coordination number of an atom is the number of atoms that are second closest to it. In a primitive cubic unit cell, the closest atoms are at a distance of 1, i.e. the edge length. Now the second closest atom to an other atom would lie at the diagonally opposite end, at a distance (2)^(1/2). The number of these atoms is 12, as there are 4 in each of the 3 planes- the plane containing the atom, the plane on top of it, and the plane beneath it. Hope this helps!