Oxidation number of cobalt ion in [CoCl2(en)2]+ will be

The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom.

In [CoCl₂(en)₂]⁺

x + 2(-1) + 2(0) = 1

x - 2 = 1 

x = 1 + 2 = 3