@geoffster Thank you for posting on the HP Support Community.
After reading your post, I did go through the feature and unfortunately, the option that you are trying to print multiple photos on one sheet using HP Smart app is not possible.
This does not mean that you will not be able to do, You can do, what you would need to do is to first download a third-party app which allows you to create a collage photos in one page.
Instagram is one option to do so, alternatively, you can search for collage app on your play store. if you search for collage app on the play store you should find many more. if your Samsung Galaxy Note 8 has the HP print plugin installed (and enabled in settings), you can print it through that.
Hope this helps! Keep me posted for further assistance.
Please click “Accept as Solution” if you feel my post solved your issue, it will help others find the solution.