Balancing chemical equations requires the same quantity of products as reactancts. There are two atoms of H on the left and therefore two on the left; one SO4 on the left thus one on the right; however while there is one Na atom on the left there are two on the right meaning to balance the equation, there must be two atoms. Therefore two moles of NaOH are required. Now there are 4 H atoms on the left but only two on the right, and 2 O atoms on the left but one on the right in the H2O molecule. Thus, to balance the equation, there needs to be two moles of H2O produced on the right. From this the equation will be balanced.