I am not sure this is the Sold out plugin that controls the products per page. I use the sold out myself and that does not influence the amount of products in my case. Are you sure there is no custom code or settings by the theme that might set this to 12?
I am using this plugin that lets you set the amount of products per page and lets your clients choose themselves how many they would like to be displayed:
You can see it in action here in one of my webshops: http://www.annadee.nl/product-categorie/kleding/blouses-tunieken/
You might also find this one usefull: