Valency of a metal

Is valency of a metal the no. Of electrons in the outer shell of an element???

Yes, that is the case for many metals like Na, K, Mg etc. However, for transition elements (like Fe, Cu etc.) show multiple valencies (leading to compounds like FeO and Fe2O3). In these elements, the electrons in the penultimate shell also participate in bond forming and can contribute to the valency.