You can't really apply simple valency rules to Manganese....
Mangenese is a transition metal, and like most transition metals, it exists in multiple oxidation states. This is due to the relative ease of electron loss compared to group I and II metals - group I metals have 1 electron in their valence s-orbital so oxidation state is almost always +1. Group II metals have two electrons in their valences-orbitals, so exhibit oxidation state +2.
However, transition metals have d-orbitals, and d-orbitals have a variety of oxidation states, as they can hold a greater number of electrons in their various degenerate orbitals. Manganese exists in oxidation states of +2, +3, +4, +6 and +7.
So the maximum oxidation number for Mn is +7, but to say it has a "valency of 7" is far too simplistic.