Return a list of 5 related products- http://shopify.com/partners/blog/93657286-4-advanced-shopify-theming-techniques-to-add-to-your-workflow

four-advanced-shopify-theming-techniques-related-product.liquid

{% assign related_collection = product.collections.first.products | sort: 'price' %}

{% if related_collection.size > 0 %}
  <div class="grid__related-products">
    <h2>Related products</h2>
    <div class="grid grid--no-gutters grid--uniform">
      {% for product in related_collection limit: 5 %}
        <div class="grid__item small--one-half medium-up--one-fifth">
          {% include 'product-card', product: product %}
        </div>
      {% endfor %}
    </div>
  </div>
{% endif %}