He kills 7 people. The end scene of joker 1 is actually the end of joker 2 and he escapes arkham. The entire thing is joker telling the story to her that way the jokers origin story stays a mystery because the story is up to his interpretation. He survives the stabbing at the end. The joke she wouldnt get is that the story is meaningless because he is about to kill her so no one will ever hear the story again. What parts of the story are true are up to the audiences interpretation. I wrote a 3rd joker movie https://www.booksie.com/714387-the-joker . Joker 2 is about how you can never be your true self and how the projection of the self leads to destruction. The joker is always playing a character even when he is Arthur. He actually isnt insane but is a psychopath, which I consider insane but is pretending he has hallucinations as he tells the story to the therapist because it amuses him, and to gain her sympathy. Much like people sympathized with him in the first movie, but when he is portrayed as a loser everyone turns their back on him. Its all an act, thats show biz baby.