Change the number of post in the Grid

Hi there, great theme you have (I have even upgraded to premium as it is a great one). I have a question though. On a category page I’d like to list more than 4 posts at a time as I only display a thumbnail as a link how can I do this? example: http://www.incredibleweightlosscenter.com/category/recipes-of-the-week/

Thank you in advance