When chlorine gas reacts with hot and concentrated NaOH solution, it disproportionates into Cl− and ClO−3 ions.
3Cl2+6NaOH→5NaCl+NaClO3+3H2O
The moles of Cl2 gas required to react with 5 moles of NaOH = 2.5 mol
So, the volume of Cl2 gas required at STP is 56 L