Slide numbers appear at the bottom of each slide. Numbers are updated when you change the order of the slides. Skipped slides aren’t numbered.
You must be in navigator view or light table view to add slide numbers.
Select the slides for which you want to show slide numbers, or select the slides with slide numbers you want to hide.
Shift-click to select contiguous slides, or Command-click to select any set of slides.
Select or deselect the Slide Number checkbox in the sidebar.
If you don’t see a sidebar, or the sidebar doesn’t have a Slide Number checkbox, click 
in the toolbar.
To show slide numbers for all slides in your presentation, choose Slide > Show Slide Numbers on All Slides (from the Slide menu at the top of your screen). To hide slide numbers for all the slides in your presentation, choose Slide > Hide Slide Numbers on All Slides.