I want to show more than 9 products per page on the collections pages. How do I do this?

Please go to Admin > Themes > Template Editor. Open up collection.liquid. Around line 12, you should see this:

{% paginate collection.products by 9 %}

This tells Shopify "show 9 products per page". Change this to a higher number; make sure it's a multiple of 3 so they display evenly. The maximum allowed by Shopify is 48.

Example:

{% paginate collection.products by 45 %}

That should show 45 products per page.