(i) Primitive cubic unit cell has atoms only at its corner. Each atom at a corner is shared between eight adjacent unit cells . four unit cells in the same layer and four unit cells of the upper (or lower) layer Therefore only 1/8th of an atom (molecule or ion) actually belongs to a particular unit cell.
since each cubic unit cell has 8 atoms on its corners, the total number of atoms in one unit cell is $\mathbf{8}\mathbf{\times }\mathbf{1}\mathbf{/}\mathbf{8}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{1}$ atom.
(ii) The atom at the body centre wholly belongs to the unit cell in which it is present. Thus in a body-centered cubic (bcc) unit cell:
(i) 8 corners$\times 1/8$ per corner atom =$\mathbf{8}\mathbf{\times }\mathbf{1}\mathbf{/}\mathbf{8}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{1}\mathbf{ }\mathbf{atom}$ 
(ii) 1 body centre atom =$1\times 1=1$
   therefore total number of atom per unit cell =2 atom.