Atomic mass of Na = 23.0amu
Density of sodium =$0.9623gcm^{-3}$
$N_A=6.023\times 10^{23}mol^{-1}$
$d=\large\frac{z\times M}{a^3\times N_A}$
$0.9623=\large\frac{z\times 23}{(430\times 10^{-10})^3\times 6.023\times 10^{23}}$
$z=\large\frac{0.9623\times (430)^3\times 6.023\times 10^{-7}}{23}$
$\Rightarrow z=2$