We set the number of posts to display on the main page for posts in the blog here > Settings > Reading
Blog pages show at most __ posts
We cannot set a different number to display on tags pages, categories pages, or Archives pages. When we publish a post it automatically displays on tags, categories and Archives pages and we cannot edit what's displayed or the number of items displayed.
In the majority of themes, posts on category or other archive pages show up just as they do on the main posts page. But in some themes only the beginnings of posts are shown, and in a few themes you only get titles or thumbnails. http://wpbtips.wordpress.com/2009/03/25/full-posts-in-archive-pages/