3. Is there a recommended number of slides for different types of presentations?
This is a familiar question, but it's the wrong question. Ten slides might be too few if they omit essential information that people can't grasp strictly from listening. But ten slides might be too many if each one is densely packed with a headache's worth of tiny text.
At the other extreme, one hundred slides are usually excessive for, say, a one-hour presentation, but not always. In one presentation I attended, every one of the nearly one hundred slides was a photograph of an amusing scene or sign that supported the speaker's key points. The speaker brilliantly synchronized his slides with his patter, resulting in a first-rate presentation.
So the right question is (assuming you've decided that slides are essential to the success of your presentation): "How can I design slides that will help me achieve the goals of this presentation?" From that question, the right number of slides evolves.
Keep in mind that gazing at slide after slide of text is an eye-straining, mind-numbing experience for the audience. Therefore, use as few text-filled slides as possible. Instead, strive for slides that feature images, such as photos, drawings, cartoons, or clip art, and little or no text.
When the information you're presenting justifies the use of bullet items, refrain from squeezing too many bullet items onto each slide. Instead, spread them across multiple slides. For example, in one of my presentations, I divide eight bullet items into four slides—two bullet items per slide—with cartoons on each slide that support and emphasize the message on that slide. Yes, I used four slides instead of one since jamming all eight bullet items onto a single slide would be unkind to the audience.
And please, please remember that listeners can't read the text on your slides and listen to you at the same time. When you present bullet items, pause and let your audience read them before you continue. My own preference is to use animation and display each bullet item only when I'm ready to talk about it.
Some people advocate avoiding text altogether (or nearly so) and limiting slides to visual images. That's fine for a presentation on your trip to the Galapagos, but for most professional presentations, I don't agree, especially if they entail technical information. Many people—myself included—take in information more effectively in text form than via spoken words or images. A little bit of text can go a long way in helping us absorb and retain information.