A chemical equation is considered to be balanced when the number of atoms of each of the elements that lie on either side of the equation are equal.
The chemical equation that has to be balanced is NaOH + SO2 --> Na2SO3 + H2O
There are two atoms of sodium on the right and one on the left, to overcome this, increase the number of molecules of NaOH to 2. This also increases the number of oxygen atoms on the left hand side to 4 and the number of atoms of hydrogen to 2. This is the same as the number of atoms of hydrogen and oxygen on the right hand side. The balanced chemical equation is:
2NaOH + SO2 --> Na2SO3 + H2O