When people keep asking "which slide was that?"
Without slide numbers, it is hard for an audience to reference a specific slide later. Adding numbers means you can say "going back to slide 5" and everyone can follow along precisely, which makes presentations and Q&A run far more smoothly.
If the number still does not show up
Applying the setting sometimes fails to display a number if the number placeholder was deleted from the slide layout itself. Check "View β Slide Master," select the layout in question, and make sure the small "#" placeholder box is still present β if it was removed, you will need to re-add it from the Master view before the number will appear.
Frequently Asked Questions
Can slide numbers start from something other than 1?
Yes. Under "Design β Slide Size β Custom Slide Size," the "Number slides from" field lets you set any starting number you want.
Can I show the date along with the slide number?
Yes. In the same "Insert β Slide Number" dialog, also checking "Date and time" adds a date to every slide alongside the number.