Just as a guideline, I’ve found that about 1 slide per minute (with not a ton of text per slide) is pretty good. This has worked for me for topics ranging from graduate level math to programming topics.
But as an addendum, your talk should not depend on the slides - the slides should illustrate or add information, trigger people's visual memory for what you said during that slide.
If you write slides with bulletpoints and read them, then er, just, don't.
