I’ve been asked by SMEs who are developing courses whether they have too many (or not enough) slides.
The real answer is that the only way to know is to pilot it. I tend to use a lot of slides, but then again I tend to use slides that build on each other with small changes. On the other hand, the right instructor with the right slide might only need the one slide, or no slides at all.
That said, I did once have a webcast moderator over several weeks track how much time instructors spent on each slide on average. The answer was 90 seconds per slide, on average.
I would never drive someone to that number, or claim that it is the “right” number of slides, but it is useful to have that in my back pocket as a point of reference when a developer has a deck that includes way more slides than that for the given time.