Last tested: May 15, 2020
For the Marketplace Image Carousel Visualization (that is not made by Looker), we can add up to 5,000 images. This is in line with how many rows we can have in the UI of an Explore, as each row will have an image.
However, the "breadcrumbs", or dots at the bottom of the visualization that we can select to navigate around, cap at 15. Any more than that, and we will use left/right. So it may not be as efficient to use more than 15, but it is possible.