Iron adopts a body centered cubic unit cell illustrate how the number of atoms per unit cell for the ion atom are obtained. So remember for any cubic unit saw we have 18 atom for every corner and that shared between 8 atoms. So for all of them it's one atom right off the bat.
But remember for a body center cubic unit cell, we also have an additional atom within the center of our cube. So we add that in. So taking those both into consideration, that's how we come up with two atoms per one unit cell for a body centered cubic unit cell.