How to change the number of posts per page in WordPress
In WordPress there are pages that display a list of posts. Such pages are the front page, category pages, archive pages, tag pages, etc. Each such page shows a fixed number of posts. By default, WordPress is set to show ten posts per page.
You can change this number using one option in the admin panel. After you log in go to Settings menu>Reading sub-menu. On the page that opens you'll see an option Blog pages show at most. Type the number of posts in the text field for that option and click on the Save Changes button at the bottom of the page.
If you need more information on the other reading settings, check out the tutorial on how to manage the reading settings in WordPress.