How do I set the Products In Category on the Categories page to show more than 20 per page by default?
You can see the full instructions at: https://docs.magento.com/m2/ce/user_guide/catalog/navigation-product-listings.html
but here's what you'll want to do:
On the Admin sidebar, go to Stores > Settings > Configuration.
In the left panel, expand Catalog and choose Catalog underneath.
Expand the Storefront section.
Best of luck!
Those settings are for the storefront only.
It is in the categories on the dashboard in the backend where the 20 per page limit is.