The electronic configuration for Co2+(z=27)=4s03d7
When it combines with weak ligands (high spin complex),t2g2,2,1eg1,1, it has 3 unpaired electrons, whereas when it bonds with strong ligands (low spin complex), t2g2,2,2eg1,0, then, it has 1 unpaired electron.
The difference in number of unpaired electrons is equal to two.