SharePoint Responsive > Web Parts > Photo or Video Gallery > Tips & Best Practices
Between three and six items/images is optimal, as more than six items will get lower interaction from the user.
Though galleries can be used with vertical images, it is better suited to horizontal (landscape) images that are between 1200px and 800px wide.
Since the gallery is responsive, it will appear differently at different viewports -- check how it responds by saving the page and narrowing and widening your browser window.
Keep captions short and simple.
Place the web part under introductory content for the best experience -- if it is at the top, it takes up a lot of space and the user doesn't see the content below it right away.