It is necessary to set the number of products shown on your home page and your various products pages (view all page, category pages, etc.) to ensure that your products fill the entire grid on each page. The correct number will depend upon your theme, your custom option settings and your own preference.

To review and adjust these settings, refer to sections 5. Set the Number of Products to Display on Your Home Page and 6. Set the Number of Products to Display on Your All Products and Category Pages of support article How do I use the basic "Customize" options for my Big Cartel theme?