How do i add the same image to multiple pages of a pdf ( Adobe Acrobat 2020) ?

Create an image field, select the image for it and then right-click it (in Prepare Form mode) and select Duplicate Across Pages. When done you can flatten the file to convert those fields to static contents. Be aware it will also flatten all other fields, as well as comments, links, etc., though.

Let me move this to the Acrobat forum for you, which is the appropriate forum for your question.

Are you asking about how to add a watermark? If so, click Tools, Edit PDF, Add Watermark.

The Using the Community forum is for help in using the Adobe Support Community forums, not for help with specific programs. Product questions should be posted in the associated product community.