Here's how to determine the number of electrons in a -2 charged anion of sulfur:

* Sulfur's Atomic Number: Sulfur (S) has an atomic number of 16. This means a neutral sulfur atom has 16 protons and 16 electrons.

* Anion Formation: A -2 charge means the sulfur atom has gained two extra electrons.

* Total Electrons: 16 (original electrons) + 2 (gained electrons) = 18 electrons

Therefore, a -2 charged anion of sulfur (sulfide ion) has 18 electrons.