A face-centred cubic (fcc) unit cell contains atoms at all the corners and at the centre of all the faces of the cube.

It can be seen that each atom located at the face-centre is shared between two adjacent
unit cells and only1/2 of each atom belongs to a unit cell.
The number of atoms present at corners per unit cell= 8 corner atoms x 1/8 atoms per unit cell = 1

The number of atoms present at faces per unit cell
= 6 atoms at the faces x 1/2 atoms per unit cell
= 6 x 1/2 = 3 atoms

Therefore, total number of atoms per unit cell = 4 atoms.