The ionic compound formed by sodium and sulfur is Na2S, whose name is sodium sulfide.
The chemical formula tells you the ratio between the two elements is 2Na/1S or 1S/2Na
So, if you have 6.4 * 10^22 sodium ions, you will have
6.4 * 10^22 Na * 1S / 2Na = 3.2 * 10^22 S.
Answer: the compound contains 3.2 * 10^22 sulfide ions.
