If a sample of this compound contains 6.4*1022 sodium ions, how many sulfide ions does it contain?

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.