Calculating from your time slot
A general rule: one slide takes about 1–1.5 minutes to present. 8–12 slides suit a 10-minute talk; 15–20 slides suit a 15–20 minute defence.
Why too many slides hurts
Too many slides (say, 40+) tire the audience and the main point gets lost. Each slide should carry one clear idea, minimal text, and a supporting visual (an image or a diagram) rather than a wall of text.
A standard structure
Title slide → outline → main slides (by topic) → conclusion → sources used → "Thank you". For a course-paper or graduation defence, a "results" slide is usually added.
Need a ready-made presentation?
OsonTopshir prepares a 10–30 slide presentation on your topic in a ready design, as a PowerPoint (PPTX) file — it can also be built from your course paper or another document.