For BCC crystal type ,there are 8 atoms at corners of the unit cell and one atom is at the centre of the unit cell.
EACH CORNER ATOMS SHARED BY 8 UNIT CELLS so the unit cell has 1/8 of corner atoms ,that is 1/8 *8=8. the atom at centre will not be shared by other unit no: of atoms in BCC is 1 +1=2