A YARPP custom template that always shows the same quantity of related posts. If there are no available posts, show random ones.

yarpp-minimum-posts.php

<?php
/*
YARPP Template: Minimum related posts
Description: Always show a specific number of posts, even if some of them are not related by YARPP
Author: @hmaesta, @pedropapa
*/

/*
 * Use as YARPP CUSTOM TEMPLATE
 * Same in your theme folder
 */
?>

<?php
$alwaysShow = 5; // Number of posts to always show

if ($related_query->have_posts()):
    while ($related_query->have_posts()) :
        $related_query->the_post();
        ?>
        <h3>
            <a href="<?php the_permalink(); ?>" rel="bookmark"
               title="<?php the_title_attribute(); ?>">
                <?php the_title(); ?>
            </a>
        </h3>
    <?php endwhile; ?>

    <?php
    $relatedCount = count($related_query->posts);

    if ($relatedCount < $alwaysShow):
        $numberToFill = $alwaysShow - $relatedCount;
        ?>

        <?php
        $loop = new WP_Query(
            array(
                'post_type' => array('post'),
                'posts_per_page' => $numberToFill,
                'orderby' => 'rand'
            ));
        while ($loop->have_posts()) : $loop->the_post();
            ?>
            <h3>
                <a href="<?php the_permalink(); ?>" rel="bookmark"
                   title="<?php the_title_attribute(); ?>">
                    <?php the_title(); ?>
                </a>
            </h3>
        <?php endwhile;
        wp_reset_query(); ?>
    <?php endif; ?>

<?php else:
// Nothing related? Show random posts!

    $loop = new WP_Query(
        array(
            'post_type' => array('post'),
            'posts_per_page' => $numberToFill,
            'orderby' => 'rand'
        ));
    while ($loop->have_posts()) : $loop->the_post();
        ?>
        <h3>
            <a href="<?php the_permalink(); ?>" rel="bookmark"
               title="<?php the_title_attribute(); ?>">
                <?php the_title(); ?>
            </a>
        </h3>
    <?php endwhile;
    wp_reset_query(); ?>

<?php endif; ?>