defining posts per page

Hi folks,

maybe there’s someone around giving a helping hand?
I’m trying to get this running.
https://www.wprecipes.com/how-to-define-how-many-post-to-display-per-page

So far I have only generated errors, after pasting this.

<?php
$page_num = $paged;
if ($pagenum='') $pagenum =1;
query_posts('showposts=7&paged='.$page_num); ?>
               	<?php if (have_posts()) : ?>
		<?php while (have_posts()) : the_post(); ?>
                // WordPress loop
                endwhile;endif; ?>

I assume this has to be inserted into the index.php of my theme? Right?
Where should I place this? Before, after, in between?
I need to have a different amount of posts on pages with different id’s.

The project I am talking about is https://www.groupshirt.net , as you can see there is a missing post on the right bottom on /?paged=2 . I want to achieve having 7 posts on https://www.groupshirt.net/?paged=1 and 8 posts on https://www.groupshirt.net/?paged=2.

Maybe there’s someone around with some knowledge about this.

Best regards.