How can I change the number of products shown on my collection pages?

To change the number of products on your collection pages, you'll need to edit either collections.liquid or collections-4-columns.liquid, depending on which one you're using.  You can edit them both to sync if you like.

Important: Make a backup first.

Then, for example, if you want to show 24 products per page:

Open collections.liquid or collection-4-columns.liquid, and change

paginate collection.products by 12

to

paginate collection.products by 24

around line 14, like this screenshot

That's it.