Answer
$1.2\times10^{18}\,units$
Work Step by Step
Mass in grams of NaCl= 0.12 mg=$0.12\times10^{-3}g$
Formula mass of NaCl= 58.44 g/mol
Number of moles of NaCl=$\frac{0.12\times10^{-3}g}{58.44g/mol}=2.0534\times10^{-6}\,mol$
Number of formula units= $2.0534\times10^{-6}\,mol\times\frac{6.022\times10^{23}}{1\,mol}=1.2\times10^{18}\,units$