Is it possible to change the number of thumbnail images on the product detail page from the default amount of 3?
Unfortunately it is not possible to adjust the number of thumbnail images on the product detail page from the default amount. That is, if the amount of visible thumbnails on the page is three, it can't be made to show four, however you can scroll to see the ones appearing next to them. You may want to consider some of the other image galleries which you can find under the Design>Themes section ("General Settings" tab, "Display tab", "Product Details" dropdown, "Image Gallery Style")
