How many atoms can be assigned to its unit cell if an element forms?
(i) a body centred cubic cell, and?
(ii) a face centred cubic cell ?

Number of atoms that can be assigned to its unit cell if an element forms,

    1.  Body centered cubic unit cell (BCC) : It has 1 atom at each of its corners and also one atom at its body            centre. So the body centered unit cell has
                         (i) 8 corners X 1/8 per corner = 1 atom     
                         (ii) 1 body centre atom = 1X1= 1 atom .
         Hence total 2 atoms.
    2.  Face centered cubic unit cell (FCC) : Contains atoms all at the corners and at the center of all the                   faces,of the cube. So total atoms are 
                        (i) ​8 corners X 1/8 per corner = 1 atom
                        (ii) 6 faces X 1/2 atom per unit cell = 3 atom
       Total 1+3 = 4 atoms.

hope you got it...! :)