I was able to find a solution:
if(!empty($search)){
$query = $wpdb->prepare(
"SELECT DISTINCT user_id
FROM wp_usermeta
WHERE meta_key = 'pie_address_3'
AND meta_value LIKE %s
", '%' . $search . '%' );
$ids = $wpdb->get_results($query); ?>
<ul class="author-list">
<?php
foreach ($ids as $id) { ?>
<li>
<img width="90" height="90" src="https://wordpress.stackexchange.com/questions/182955/<?php echo get_user_meta($id->user_id,"pie_profile_pic_5", true); ?>"
class="attachment-agent-image wp-post-image">
<h2 style="display: inline; margin-left: 20px;">
<a href="<?php echo get_author_posts_url($id->user_id); ?>"><?php echo get_the_author_meta('first_name', $id->user_id); ?>
<?php echo get_the_author_meta('last_name', $id->user_id); ?>
</a>
- <?php echo count_user_posts($id->user_id); ?> immobili
</h2>
</li>
<?php } ?>
</ul>
<?php } ?>