In 3d series of transition metals manganese shows largest number of oxidation states. The outer electronic configuration of manganese is 3d54s2. As 3d and 4s are close in energy, it has maximum number of electrons to lose or share because it has highest number of unpaired electrons in its d-orbitals.
So, by removing its all electrons we get different oxidation states. It shows oxidation states from +2 to +7 which is the maximum number.
Example: MnO2(+4), MnO−4(+7), MnO(+2), etc.