Question Regarding Formulas

A question that I must answer showed me a 3d model of a sodium chloride molecule. I had to count each ion for both Na and Cl. (In total there were 18 Na ions and 18 Cl ions)

It says I must write a formula that will represent each Na and Cl atom shown in the model but I am not sure how to do that. Would the formula be just NaCl or something else because of the amount of ions. Also, how would you double that formula?

I can't find anything that will help answer :/