How to limit the number of Related Posts?

You need the argument post_per_page
to limit the posts.

Before your loop it should be something like this.

$args = array (
    'post_type'              => 'custom_post',
    'posts_per_page'         => '10',
);

$wp_query = new WP_Query( $args ); 


// your loop 
if ( $wp_query ->have_posts() ) {

    while (have_posts()) : the_post();
      get_template_part( 'preview', get_post_format() );
    endwhile;

}

wp_reset_postdata(); 

If it is the custom post, make sure to replace custom_post