Each lattice point in a body-centered cubic (bcc) crystal structure is shared by eight nearby unit cells. In a bcc lattice, the {111} plane connects the opposite corners by passing through the cube's center. It is customary to indicate the quantity of atoms on this plane as 0.5 atoms per unit cell.
This is due to the fact that there is one atom at each corner of a bcc lattice, and each corner atom contributes 1/8 of its volume to the unit cell it resides in. As a result, each unit cell from the corner atoms has approximately 1 atom * 1/8 = 0.125 atoms.