Seth Godin is fantastic not only for his marketing expertise, but for his incredible understanding of the importance of effective presentation design. He even wrote one of the earliest e-books on presentation design called "Really Bad PowerPoint (and how to avoid it)". He has written a number of posts about the topic as well (just search "PowerPoint" on his blog). Of all the live presentations I've ever seen, Seth's Linchpin presentation was the best - which is saying a lot considering I've seen both Garr Reynolds AND Nancy Duarte live.
Seth recently wrote a post entitled "The 200 Slide Solution" that addressed the need to separate your ideas onto multiple slides, possibly even 200 for a 40-minute presentation (one every 12 seconds). Seth writes:
You're used to putting three or four bullet points on a slide. That's at least four distinct ideas, but more often, each of those ideas has three or four sub ideas to it. In other words, you're cramming 32 ideas on a slide, and you're sitting on that slide as you drone on and on.
How would your pace change if you had 200 slides? How much better would the integration of slides and talk be?
I don't honestly expect you to do your presentation with 200 slides. I'm hoping this exercise will help you realize that you might not need any slides. Or that 50 or 100 slides will pick up your energy and make your argument more coherent.
Seth is right on point. Not only are bullet-points awful when it comes to engagement and information recall, but their use often leads to slides filled with text and numerous different ideas. Don't be afraid to give your slides some breathing room!!
There's no "optimal" number of slides in your presentation. It's not about numbers. It's about crafting an effective story and utilizing your visuals to enhance and amplify that story. However, don't use that as a crutch for using too few slides and cramming information into every crevice. Find a way to give each important idea its own slide. I separate each idea onto one (or many) slides and love the faster pace, continual re-engagement, and visual rewards that it provides.
What's your feeling about the optimal number of slides? What pace do you like to present at?