Posts Per Page Question

Hi. Sorry if someone already posted about this, but I didn’t see it.

I am working on a site for a client. I have a child theme of Snapshot and I want to change the number of posts per page in our blog (http://trentonvotes.com/category/blog/). Our blog is setup as a category page. I want to change the number of posts on the page to 12. What file would I change to do this and how would I incorporate it into my child theme?

Thanks!