How to display how many have been sold by Jenny Ly

Hi, I'd like to add a feature that is to have a goal of for example selling 150 items of certain product and showing how many have been sold. This needs to be easy to manage through the admin. One important thing is that only some products are going to have this feature, so it needs to be conditional. Example:

http://teespring.com/lastxbmcshirt1

On the bottom right, it says

1057 sold towards goal of 150

1057 have been sold (towards the original goal of 150)
You can keep buying until the campaign ends

I found this topic that seems relevant, but can't figure out how to modify it:

https://ecommerce.shopify.com/c/ecommerce-design/t/how-to-display-variant-inventory_quantity-46970

Thanks!