1, With your PrestaShop admin panel, go to Modules -> Module Manager
2, Choose the modules tab and type in the Search field “Products in the same category” to find the module. and Click on the Configure button.
3, Set the number of products and click on the Save button.
PrestaShop 1.7.x – How to create a Cross-selling Product #
PRESTASHOP 1.7.X – HOW TO CREATE A RELATED PRODUCT #