Slide numbers in Google Slides

People using Google Slides can now add slide numbers to their presentations, providing a reference guide for both presenters and audiences. To add slide numbers, go to Insert > Slide numbers from the toolbar. You can then choose to apply slide numbers to all slides or selected slides. You can also choose to skip title slides:

 

Release track:
Rapid release, with Scheduled release coming in two weeks

For more information:
Help Center
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Reference by www.google.com