You can add the Keynote equivalent of a page numbera slide numberto any slide. Open the Slide Inspector's Appearance tab and turn on the checkbox for Slide Number. Keynote inserts the slide number, centered, at the bottom of the slide as a free text box. You can reformat the number or reposition this text box just like any other free text box. But unlike other text boxes, you can't add text or replace the numbersince Keynote updates the number if you move the slide to a different position in the show. If you want your slide number to read, "Slide number 5," for example, you have to add another free text box to hold the words "Slide number."
If you want to use slide numbers throughout your presentation, you probably want to add the slide number to your slide masters, as described on Section 12.3.