You should use just enough slides to tell your story, and no more. The preceding question and its answer are pretty close an old joke. When Abraham Lincoln was asked: “How long should a man’s legs be?” he replied “Long enough to reach the ground.”
How many slides should you present per minute? You can just follow Spiegel’s Law, which states that the answer to any question will turn out to be “about three” when the question is asked in the correct units. A guideline of “1 to 2 slides per minute” is often stated, but this is for the mythical “typical” slide. Obviously it matters whether you have zero, five, or fifty words on a slide. The Global Health Education Consortium makes the sensible suggestion that you should: “consider 1 – 2 slides per minute for those that have text or graphic content, and perhaps up to 3 – 4 per minute for photos”.
The Director of Media Services at the Oshkosh campus of the University of Wisconsin, Nick Dvoracek, gave a 12 minute Macromedia Breeze talk on Effective Presentations which has 41 slides, or 3.4 per minute. His web site also contains much more information about using PowerPoint.
Is it possible to pack even more slides per minute into an effective presentation? How about over TWICE as many as Nick Dvoracek? Earlier this year Alvin Trusty gave a 45 minute presentation with 327 slides (7.2 per minute). At TeacherTube you can view a video of his amazing “Beg, Borrow, but Don’t Steal (How to give a great PowerPoint without breaking the law).” While Alvin seems a bit rushed, he still is perfectly understandable.
How many still images per minute can be presented for the very special case of a music video, where the song tells the story? The video for the Carrie Newcomer song Angels Unaware has over 115 images in just 4-1/2 minutes. That is over 25 “slides” per minute, although these images are enhanced by “pan and zoom” camera movements (in the same style Ken Burns employed for his documentary on the Civil War).