This is a triatomic, linear molecule so it has 3N-5 modes-- so 4. It will have symmetric stretch, asymmetric stretch, and 2 degenerate bending modes. As a tri-atomic, linear molecule, the asymmetric stretch and bending modes will be Raman-inactive, leaving only the symmetric stretch for Raman.

I agree wit Camp answere

Crystalline CeO2 has 3 atoms/unit cell and crystallizes in the fluorite-structure lattice, space group Oh5. With 3 atoms/unit cell there are 9 degrees of freedom to be accounted for: 3 acoustic modes and 6 optical modes. The optical modes comprise 1 nondegenerate LO mode, 1 doubly degenerate TO mode, and one triply degenerate (Raman active) F2g mode. The symmetry of the crystal determines the number of Raman-active modes.