wp-docs

Edit the file called “YourWordPressFolder/wp-admin/includes/post.php”

Look for the code below (should start around line 529)
function wp_edit_attachments_query( $q = false ) {
global $wpdb;
if ( false === $q )
$q = $_GET;
$q['m']   = (int) $q['m'];
$q['cat'] = (int) $q['cat'];
$q['post_type'] = ‘attachment’;
$q['post_status'] = ‘any’;
$q['posts_per_page'] = 15;

Change $q['posts_per_page'] to whatever you wish.

The default value for $q['posts_per_page'] is 15.