How many iron atoms are in the iron oxide?

Depend on it's valence state. Iron is transition metal and had many possible valence state (e.g. Fe2+ or Fe3+). For the case Fe2+ then it is just FeO and 1 atom informula but if it is Fe3+ then it is Fe2O3 and thus 2 atoms. You must check what written clearly, for iron oxide such as rust, it is wrtten as iron(III)oxide to stand for being a Fe3+.