I think All those murders and killings were all Imaginations of Mr P. Bateman as he referred to the last line of the movie saying "this confession has meant nothing." If he had really killed so many people than all his confessions would have meant something...like he is insane ,,,a psycho.
P. Bateman was also a coke addict as we could see from the movie. So may be his mind was in a different state and maybe he was taking trips..maybe!
Also from the reality, if he had really commited so many murders then some evidences should have come up..and it is also impossible to keep so many bodies in a home due to smell problem..so i think it was all in his head only.