- Select View > Header and Footer...
- The option to number your slides is found in the Header and Footer dialog box.
- Select options in the Header and Footer dialog box, such as adding a date and time to the presentation and whether you want slide numbers to show on each slide. Most presentations do not show a slide number on the title slide, so you may want to check the option "Don't show on title slide".
- Choose "Apply" or "Apply to All"
- Slide numbers will now appear in the bottom right corner of the slide(s).
- Access the slide master to change the font size and style of the number.
For more information on using slide masters in PowerPoint see this related article Custom Design Templates and Master Slides in PowerPoint