The oxidation number of Mn in KMnO₄ is +7

Explanation:
We know that the oxidation numbers should add up to 0.

So in KMnO₄, Pottasium has an oxidation number of 1 and Oxygens is -2.

Let us say that the oxidation number of Mn is x

So, taking the number of atoms into consideration

1(K) + x(Mn) + (4 * (-2)) (4 atoms of O) = 0
=> 1 + x – 8 = 0
=> x or Oxidation number of Mn = 7