In the synthesis reaction where two atoms of sodium (Na) combine with one molecule of chlorine gas (Cl2) to produce sodium chloride (NaCl), the balanced chemical equation is:
\[ 2 , \text{Na} + \text{Cl}_2 \rightarrow 2 , \text{NaCl} \]
This shows that for every molecule of chlorine gas, two molecules of sodium chloride are produced. Therefore, the answer is:
two