Jekyll post content for adding some code after regular instances.

post.html

...
<div class="entry">
  {% include share-bar.html %}
  {% assign content_para = content | split: "<p>" %}
  {% for para in content_para %}
    {{para}}
    {% assign para_number = forloop.index | modulo: 10 %}
    {% if para_number == 0 %}
      <h2>Your SPECIAL CODE</h2>
    {% endif %}
  {% endfor %}
</div>
...