In CsCl lattice, the coordination number of $C{s}^{+}$ ion is 

Each ${\mathrm{Cs}}^{+}$ is surrounded by eight ${\mathrm{Cl}}^{-}$ ions in CsCl crystal lattice because its co-ordination number is 8 : 8.It follows 8 : 8 type of arrangement. Radius ratio (r_c+/r_a-) of CsCl is 0.93 .