For the reaction: I ^ - + Cl O _ 3 ^ - + H _ 2 S O _ 4 → C l ^ - +...

The balanced equation is,
$Cl{O}_{3}^{-} + 6{I}^{-} + 6{H}_{2}S{O}_{4} \to 3{I}_{2} + C{l}^{-} + 6HS{O}_{4}^{-} + 3{H}_{2}O$

Here, in this reaction,

${HSO}_{4}^{-}$ is in one of the products, and by balancing the reaction, we get its stiochiometric coefficient as 6.

Oxidation number of iodine in reactant side is $- 1$ and it is oxidised to ${I}_{2}with0$ oxidation number in product side.