The total number of valence electrons in an atom of Mg-26 in its ground state is two (2).

The ground state of any element is that state at which it possesses the least amount of energy. In this state the attraction between the nucleus and the surrounding electrons is sufficient to prevent electrons from escaping.

Answering the question:

In order to correctly answer this question we need to understand the meaning of the term valence. Valence refers to the number of electrons available to bonding found in the outermost orbital of an element. The above diagram has 2 electrons present in its outermost orbital, hence the total number of valence electrons is two. The inner negatively charged electrons are physically closer to the positively charged nucleus and result in very tightly packed inner orbital of electrons.