WordPress Post Category Page layout

I would like to be able to customize the layout of these pages but have no idea where to start or how to do so. I want to be able to have 20 posts on one page and be able to have a 1234 etc button below. Currently the only way to do it is to keep pressing the previous or next buttons. I want to be able to customize how many posts show up on one page and what the layout looks like.

Thanks

The page I need help with: [log in to see the link]