In an acidic solution the MnO4^- goes to Mn^2+ which is a gain of 5 electrons per mole MnO4^-. S in sulfite is 4+ and it changes to 6+ in sulfate which is a loss of 2 electrons/mol. After multiplying the Mn by 2 and the sulfite by 5 the total electons changes = 10.
So the reduction half (the MnO4^-) gains 5e/mol or 10e/2 mols in the balanced equation.