Valence Electron Question

I have a vague idea about what valence electrons are, but I am kind of confused. They are supposedly the electrons in the outermost energy level, but does this include electrons in the d sublevels?

For example, Ni: [Ar] 4s2 3d8
Would Ni have 10 or 2 valence electrons?

It has 10. Ni = [Ar] 3d8 4s2 and if you count the number of valence electrons in each level, it comes up to ten.