Slide Numbers Appear on Slides
Slide numbers will now automatically appear on all PowerPoint slides in the bottom right corner. One nice feature about letting PowerPoint number your slides is that if you rearrange the slides into a different order, PowerPoint renumbers the slides to display the new correct slide number.
The font size and style of the number and other options can be edited by accessing the slide master.
For more information on using slide masters in PowerPoint see this related article Custom Design Templates and Master Slides in PowerPoint