+2 is the oxidation number for Zinc
Oxidation Number for a Transition metal = Number of Unpaired d-electrons + Two s-electrons
[Ar]4s2 3d10 is the electron configuration for zinc. The d-block is full with 10 electrons meaning there are no unpaired d-electrons, so the oxidation number is 0 unpaired d-electrons + 2 s-electrons = +2