How many slides should you include in a PowerPoint presentation? Here's the rule of thumb I would use. Ask yourself for every single slide. Number one, does this make an important point I have more memorable to the audience than me simply stating it? And number two, does this make the point I'm trying to convey more understandable than me simply saying it? If the answer's not yes to both of those, throw the slide away.
Here's the other big test for a PowerPoint. Let's say you're giving a PowerPoint presentation to 50 clients on Thursday. Find 3 colleagues in your office. Practice your speech Tuesday. Give them your whole PowerPoint presentation. When you're done ask them every slide they remember. Any slide they can't instantly remember, and describe to you, and tell you what it was and what the message was, you now have empirical evidence that that slide was completely worthless. Take that slide and throw it in the trash can. Or, use it as a handout before the speech or after. It's just not working. If your audience can't remember it it's not working.
You've got to ask yourself not about how many slides you need. Your goal in a presentation is not to deliver slides. Your goal in a presentation is to convey ideas and get people to remember your ideas. Slides are simply one tool to increase the odds that people remember and understand your ideas. So, don't think of it as a rigid, you must have 7 slides or 10 slides or no more than 5. You could have 2 slides but they're incredibly awful and useless. Or, you could have 20 slides and each one is memorable and really drives home a point.
So, the answer isn't the number. The answer is does each slide make it more understandable and more memorable.