How many slides are appropriate will depend on your presentation style, and what is on your slides. In a web page by Doug Lowe about his PowerPoint 2013 for Dummies book he said to:
“Try to shoot for one slide for every two to five minutes of your presentation.”
That’s just 0.2 to 0.5 slides per minute. It might apply to slides containing several lines of text. But, slides with just a few words or an image that pass the glance test described by Nancy Duarte can be presented at much higher rates. For example, there are brief presentation formats such as Pecha Kucha (3 slides per minute) or Ignite (4 slides per minute).
Larry Lessig is an example of someone whose style effectively uses lots of slides. Watch his TED talk on We the People, and the Republic we must reclaim.
What usually doesn’t work is those dual-purpose slides that Garr Reynolds called slideuments. They remind me of Shimmer, a 1976 product in a phony commercial on Saturday Night Live, that tried to be both a floor wax and a dessert topping. Slideuments are a set of slides that are used in a presentation, but also are meant to be handouts - written documents read later and much more slowly.