The answer is; 2.
To balance a chemical equation, the moles on one side of the equation has to be the same as that on the other side. This ensures that the law of conservation is observed because matter or energy can't be created or destroyed but can only be transformed from one form to another.
In this equation, putting 2 in front of NaCl ensures that there are 2 moles of Na and CL just as there are 2 moles of Na and CL in the reactants side.
