Question

12 protons, 10 electrons, 10 neutrons is that Mg+2?

Answer 1

We know that it's an atom of magnesium, since this atom has 2 protons.
However, it is an ion, which means it doesn't have the same amount of electrons as protons. A neutral atom of Mg will have 12 protons and 12 electrons.

Since this atom is of Mg2+, that means the atom has a net charge of +2. Therefore, the atom must be missing 2 electrons. An atom of Mg2+ indeed has 12 protons and 10 electrons.

About the neutrons, it doesn't matter which isotope of magnesium it is.