PowerPoint mistakes that drive people crazy
Too many slides; too few slides
How many should you use? I usually say 10, only because it's a finite number, and it's less than 1,000.
A better answer: It depends. It depends, for example, on whether your audience can see you. If so, use fewer slides -- or none.
But if you're leading a virtual meeting, and you're invisible to the audience, change your slides every minute or so to keep things moving. The other day, I used 28 slides for a 90 minute webinar. It worked.