The primitive cubic system consists of one lattice point on each corner of the cube. Each atom at a lattice point is then shared equally between eight adjacent cubes, and the unit cell, therefore, contains in total one atom.
The body-centred cubic system has one lattice point in the centre of the unit cell in addition to the eight corner points. It has a net total of 2 lattice points per unit cell.
The face-centred cubic system has lattice points on the faces of the cube, that each gives exactly one-half contribution. In addition to the corner lattice points, giving a total of 4 lattice points per unit cell.
For simple cubic, number of atoms per cell = 1
For bcc, number of atoms per cell = 2
For fcc, number of atoms per cell = 4