How do you insert slide numbers on powerpoint slides?

simply INSERT -> SLIDE NUMBER, option's there for user comfort.

OR

1. In the pane that contains the Outline and Slides tabs, click the first slide thumbnail in your presentation.
2. On the Insert tab, in the Text group, click Slide Number.
3. In the Header and Footer dialog box, do one of the following:
   • To add slide numbers, click the Slide tab, and then select the Slide number check box.
   • To add notes page numbers or handout page numbers, click the Notes and Handouts tab, and then select the Page number check box.
4. To add page numbers to all of the slides, notes pages, or handouts in your presentation, click Apply to All.