The number of Fe atoms within a single FeCl2 molecule are one.
Explanation:
Metallic iron produces only ferrous chloride (FeCl2) on treatment with hydrochloric acid. Hydrogen gas is also liberated as the other product. But when iron is heated with chlorine gas, ferric chloride (FeCl3) is formed.
Total number of atoms within a single FeCl2 molecule are 3 atoms.