Go here - http://YOURDOMAIN.COM/wp-admin/options-reading.php and change the number of posts you want to show on your archive pages

Replace YOURDOMAIN.COM with the name of your domain..

If that does not do it for you , then look for any function in the functions.php file of your active theme and either delete/comment that function or change the number of posts that function is set to show on your archive pages ....

If you cant find the code, then use this http://pastebin.com/ to share your functions.php....