Answer is: 4 atoms of nitrogen.
One formula unit of magnesium nitrate (Mg(NO₃)₂) has two atoms of nitrogen, so two formula units of magnesium nitrate will have four atoms of nitrogen (2·2=4).
Oxidation number of magnesium in magnesium nitrate is +2 and oxidation number of nitrate anion is -1 (2·(-1) = -2).
