In the synthesis reaction where two atoms of sodium (Na) combine with one molecule of chlorine gas (Cl2) to produce sodium chloride (NaCl), the reaction can be represented as:
\[ 2 , \text{Na} + \text{Cl}_2 \rightarrow 2 , \text{NaCl} \]
From the equation, you can see that 2 sodium atoms combine with 1 molecule of chlorine to produce 2 molecules of sodium chloride. So, the answer is two.