Number of atoms per unit cell,if atoms are present at the corner of unit cell and two atoms at each body diagonal

There are 4 body diagonals in a cube.

There are two atoms in each diagonals.

Hence total no. of atoms atoms on the diagonal is 4×2=8

These 8 atoms are within the body of the cubic unit cell and are not shared by any other units.

The contribution of 8 corner atoms 8×1/8=1

thereforetherefore total no. of atoms in the unit cell = 8+1 = 9