Sometimes it’s useful to have the slide number on the presentation slide itself, rather than just on the handout. This is easy to add.
On the Insert tab, Text group, you’ll see the Slide Number button. The dialog box it brings up can be a little confusing. Look toward the bottom and find the check box for Slide number. You can also add other elements to your presentation slide here, such as the date and time or even the footer that you find in the Slide Master.
Click Apply or Apply To All as needed.