wp-docs

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

Look for the code below (should start around line 122)
$pagenum = absint( $_GET['pagenum'] );
if ( empty($pagenum) )
$pagenum = 1;
if( !$per_page || $per_page < 0 )
$per_page = 20;
Change $per_page from the last row to whatever you wish.

The default value for $per_page is 20.