The coordination number of a molecule is referred to as the number of ions of the other charge surrounding the ion of the opposite charge. In the case of sodium chloride we have sodium ions placed and there are 6 chloride ions surrounding each of the sodium ions. In the case of NaCl the ratio is 1:1 and as a result there are 6 sodium ions surrounding the chloride ion and vice versa.
In case of the periodic table we are well aware that as we move down the group then the size of the cation increases and as a result of the increase in the number of the shell as we move down. Thus we find that the size of the ion increases and hence caesium has a very large size compared to that of sodium. Hence there is a change in structure: the cesium ions do not form a lattice similar to that of sodium but rather we find that it occupies the centre of the cube that there are 8 chloride ions surrounding it. Also as the ratio of CsCl is also as that of sodium hence we find that the coordination number remains as 8 in this case.
Hence the answer to this question is that the coordination number of cesium in cesium chloride is 8.