the balanced chemical equation for the above reaction is as follows;
2Na₂CrO₄ + 2HCl ---> Na₂Cr₂O₇ + 2NaCl + H₂O
stoichiometry of Na₂CrO₄ to NaCl is 2:2
the amount of moles of Na₂CrO₄ reacted is equal to the number of moles of NaCl produced.
We are told that Na₂CrO₄ reacts completely, this means that this is the limiting reactant , amount of product forms depends on amount of limiting reactant present.
When 2 mol of Na₂CrO₄ is used up , according to stoichiometry number of NaCl moles formed are equal to that.
Therefore number of NaCl moles formed - 2 mol