Find slides needed for a given talk length.
Slide count follows from speaking time, not slot length — subtracting Q and A first is the step most people skip, and it is why talks overrun. Dividing the remaining minutes by your target pace gives the content slides, with title and closing added separately since they take almost no time. Roughly one slide per minute to ninety seconds is the conventional pace for a technical talk.
Speaking time
Speaking minutes = slot length - question and answer minutes
Slide count
Content slides = round(speaking minutes / minutes per slide); total = content slides + title and closing
With five minutes of questions, about twelve to fifteen content slides at a one-minute pace. Denser slides mean fewer; a rapid visual style can support many more.
Because slide count is set against the whole slot rather than the speaking time, and because rehearsal is faster than delivery — questions, technical fumbles and unplanned asides all add minutes.