No of atoms on corners: 1/8*8 = 1
No of atoms on faces: 1/2*6 = 3
The number of atoms in FCC unit cell is the sum of no of atoms on corners and faces i.e., 1+3 = 4.
A slip system is composed of a slip plane and a slip direction. It occurs along the close-packed plane. Therefore, in FCC crystal, the number of slip systems is 12