The FCC - crystal contains how many atoms in each unit cell?

We know that he face-centered cubic $\left(\mathrm{FCC}\right)$ unit cell can be imagined as a cube with atoms present on each corner as well as on each face centre. 

Now, atom at each face centre is shared with the other adjacent face centre and also each corner is shared with eight other adjacent corners making it sophisticated packing lattice.

Thus, contribution of atoms in $\mathrm{FCC}$ lattice will be:

$\Rightarrow 6\times \frac{1}{2}+8\times \frac{1}{8}$

Hence, the effective number of atoms is $4$.