The number of cards will depend on (1) the amount of content in the lecture, (2) the granularity of your questions, and (3) your selection filter.
You could easily produce 100 cards/lecture in a typical class. Many students are turned off by this number, but the fault is not Anki's. That's just the honest truth about how many discrete facts there are to be remembered, and Anki (or physical flashcards, notes, w/e) just makes it obvious how much there is to know.
The important question here though, should you try to Anki everything? the answer is, it depends on your goals and motivations. If you're using Anki (or any other SR app) for long-term retention and longer-term tests, then it might not be worth your time to get everything into Anki from lecture slides, since a good fraction is likely low yield and "forgettable".
A good rule of thumb is: If you want to remember something beyond a test, put it into Anki. If you want just get something into your head for an upcoming exam, cram it. Don't waste your time making cards for stuff you're OK forgetting.
You can still take advantage of the techniques that underlie Anki (spacing and testing) for your cramming. the effects just won't be very lasting. So, even for your cramming, rather than running your eyes over slide after slide, try to reproduce from memory, with no prompting, everything you remember from a lecture. This process of retrieving knowledge is very potent, even in one or two repetitions. Indeed, much of the literature that looks at the testing effect use retrieval practice just in the short term, and
it still produces significant effects. So, even your cramming can be "Anki-like" without having to make cards. Save the cards for the things that matter (i.e. not every piece of minutiae your prof wants you to regurgitate).
For a more comprehensive discussion about knowledge filtering and Anki, you can check out
Learning Medicine: An Evidence-Based Guide (which I co-authored).