When I’m reviewing presentations I always advise presenters to decide the three most important things that they want their audience to take away with them. Then they should work backwards to create a presentation that will help their audience remember at least one of those three things.
Years ago I remember learning about the psychologist George Miller quantifying short term memory in the paper The magic number seven, plus or minus two. He arrived at this number by testing subjects in a lab; I believe if he tested them with a PowerPoint presentation at a meeting the magic number would have been three, plus or minus two.