Well, I figured out how to do it by myself. Maybe it's not the best solution, but it works.
So you have to edit this file: modules\mod_sj_k2_responsive_content\tmpl\default.php
Go to the line 179 and find this code:
<?php if($params->get('itemCommentsCounter',1) == 1) {?>
<span class="item-comment">
<?php echo $item->numOfComments; ?>
<span class="item-comment-bottom"></span>
</span>
<?php }?>
Replace it with:
<?php if($params->get('itemCommentsCounter',1) == 1) {?>
<span class="item-comment">
<?php
/**
* This section was added to enable use of JComments in Sj Responsive Content for K2
* Remember to install K2 JComments plugin first!
*/
$commentsAPI = JPATH_SITE . '/components/com_jcomments/jcomments.php';
if (is_file($commentsAPI)) {
require_once($commentsAPI);
$count = JComments::getCommentsCount($item->id, 'com_k2');
echo $count;}
else {
echo $item->numOfComments;} ?>
<span class="item-comment-bottom"></span>
</span>
<?php }?>