Researchers (Young, Lareau, and Pierre) calculated that in the DSM-5 there are 270 million combinations of symptoms that would meet the criteria for both PTSD and major depressive disorder.
Reference: G. Young, C. Lareau, B. Pierre (2014). One quintillion ways to have PTSD comorbidity: recommendations for the disordered DSM-5. Psychol. Inj. Law, 7 (2014), pp. 61-74.