In PowerPoint, it’s hard to remember what slide number you are presenting. As an aid, we can add the slide number to our slides.
One way, we could manually add a number using a text box. But what if you added or deleted a slide? This would require you to manually update your slide numbers; a time consuming and error prone task.
Instead, you can have the slide numbers update automatically on all your hidden and active slides. By default, your slide numbers appear in your slide footer, but you can move and format them.
This is for PowerPoint for Office 365 Windows and Mac versions. Other versions of PowerPoint will be similar.

Adding Slide Numbers
Open a PowerPoint presentation with several slides to see the feature. Click the Insert tab. In the Text section, click the Header & Footer icon. See below image:

The Header & Footer options box will open. Refer to below image:

Under the Slide option, place a check in the Slide number checkbox. To add your slide number to all of your slides, select Apply to All button. Refer to below image:

Your slide number will appear in the bottom right corner. Note! The number place holder will be small. See following image:

Remove Slide Numbers
To remove your slide numbers, follow the steps above. In the Header & Footer options options box, simply uncheck the Slide number checkbox. Then click Apply to All button. Your slide numbers will be removed.
Format Slide Numbers
We are going to use PowerPoint Slide Master to make the slide number in a different font, color, size or position.
To do this, click View from the main menu, then select Slide Master. This will load the Slide Master editing box. You will see the slide number box in its default position at the bottom right section of the slide. See following image:

Note! You could add a text box with the word “Slide” next to the slide number; this is not shown.
When you move or apply a different font, size or color, it will apply to all your slides.
To edit the text box, select the box then select the Home tab. See below image:

Then you can use the available options in the Font and Paragraph sections to format the number like choosing Bold, Underline, etc. I have set the number to be Bold and made the font size bigger. See below image:

Once you have finished editing, return back to the Slide Master tab and click Close Master View button. Refer to below image:

Be sure to Save your presentation with the automatic slide numbering feature.
Related
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How to setup a PowerPoint presentation using the Loop feature
How to add Rolling Credits to your Powerpoint presentation
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