‘posts_per_page’ in costum page type

Question

0 isn’t a valid value for posts_per_page. WordPress will see it as empty and default to the posts_per_page option. If you want all the posts use nopaging or posts_per_page => -1.

$query = new \WP_Query([
  'post_type' => 'producenter',
  'nopaging' => true,
  'orderby' => 'rand',
  'post__not_in' => [$post->ID],
]);

As far as why it’s only showing 1 post, is that what you have set in the default posts per page option? Not sure why it would only show one post if you set posts_per_page to 10.