How can I show only 5 posts from the relationship?

You could use the key from the foreach

<?php
$c_lists = get_field('c_lists');
if ($c_lists) :
    foreach ($c_lists as $key => $post_ids) :
    if ($key > 4) break;
?>
<a href="<?= get_permalink($post_ids); ?>"><?= get_the_title($post_ids); ?></a>
<?php
    endforeach;
endif;
?>