Answer
4 $Fe$ atoms per hemoglobin molecule.
Work Step by Step
Iron in hemoglobin makes 0.35% of mass, so if we have $x$ atoms of $Fe$ in hemoglobin, we can conclude that
$x\times Ar(Fe) = 0.35\% \times M(hemoglobin) = 0.0035 \times 65000 \frac{g}{mol} = 227.5 \frac{g}{mol}$. Hence, $x = \frac{227.5 \frac{g}{mol}}{56\frac{g}{mol}}\approx 4$.
