Why do you need two ammonium ions and one sulfide ion for the formula for ammonium sulfide?

Ammonium (NH4) + gains one additional electron to form it's 1 + charge state. Sulfur, like oxygen, gains two electrons per atom to ionize to a 2 - charge state.

To form an electrically stable ammonium sulfide, two ammonium ions combine with one sulfide ion. (NH4) 2S