The coordination number of a transition metal complex refers to the number of ligands directly bonded to the central metal ion. In the given complex [Co NH3 4Cl2]Cl, the central metal ion is cobalt Co .There are 4 ammonia NH3 ligands and 2 chloride Cl ligands directly bonded to the cobalt ion. Therefore, the coordination number of this complex is 4 + 2 = 6.