With some WordPress themes, the theme overrides the ability to increase the number of products per row in the archive pages.
This plugin, written by James Koster of the WooCommerce team, is pretty old but works OK – at least for now. As it describes:
It allows you to customise WooCommerce product archives. Change the number of product columns and the number of products displayed per page.
Toggle the display of core elements and enable some that are not included in WooCommerce core such as stock levels and product categories.