Spoiler: Why I think he only killed..

..four women (three girls from accounting department and assisted suicide of his mother).

I read somewhere that most serial killers kill on accident for the first time. Then they trying to replicate the feeling and it never matched it so they are repeating it over and over.

I think that especially the first part matches. He kills on accident and then he feels urge to kill again (via his voices). The mother episode (and his abusive father) was just something that pushed him to be like this and got direction to his mental illness - she had the same thing but probably did not killed anyone.

There was no evidence that he does not remember things. His view of reality is just skewed. Therefore I do not think that he killed before and forget it and there are countless of tuperware filled with body parts and heads stashed in bowling alley. It was just remains of food, pizza and other garbage. We got to see his whole evolution as killer and its escallation (also described in articles about serial killers) not just and window into it.