Refers to blog post: http://freakdesign.com.au/blogs/news/17335317
First create a new snippet called product-sold-count.liquid.
In that file copy and paste the below code:
{% comment %}
Show current amount sold for a single product (with default variant).
This assumes that you have created a metafield
{% endcomment %}
{% assign productStartCount = product.metafields.stock.initial | times:1 %}
{% if productStartCount > 0 %}
{% assign productInventory = product.variants.first.inventory_quantity %}
<p>{{ productStartCount | minus:productInventory }} sold. Only {{ productInventory }} remain.</p>
{% endif %}
On your product page you can now just include the snippet any place that makes sense:
{% include 'product-sold-count' %}
Now if want to avoid showing negative stock numbers on the page (and just show a message) you can wrap the code in a conditional statement.
{% comment %}
Show current amount sold for a single product (with default variant).
This assumes that you have created a metafield.
Message will show if variant is sold out.
{% endcomment %}
{% if product.variants.first.inventory_quantity > 0 %}
{% assign productStartCount = product.metafields.stock.initial | times:1 %}
{% if productStartCount > 0 %}
{% assign productInventory = product.variants.first.inventory_quantity %}
<p>{{ productStartCount | minus:productInventory }} sold. Only {{ productInventory }} remain.</p>
{% endif %}
{% else %}
<p>Uh Oh! All sold out...</p>
{% endif %}