In the NaCl structure, the number of second nearest neighbours (same type of ion) of a given $\mathrm{Na}{}^{+}$ ion is

$\mathrm{Na}{}^{+}$ ions form an fcc lattice. In an fcc lattice each lattice point has 12 nearest neighbours of the same kind, at distance $a/\sqrt{2}$. These are the second neighbours of the central $\mathrm{Na}{}^{+}$ (the first shell being the 6 $\mathrm{Cl}{}^{-}$).