A neutral atom of magnesium contains 12 protons, 12 neutrons, and 12 electrons.
To break this down: the atomic number of magnesium is 12, which means it has 12 protons in its nucleus. In a neutral atom, the number of electrons equals the number of protons to balance the charge, so magnesium also has 12 electrons. The number of neutrons can vary in isotopes of the element, but the most common isotope of magnesium has 12 neutrons. Therefore, for a stable, neutral atom of magnesium, you will find 12 protons, 12 neutrons, and 12 electrons.