Not interested in a third party app it should already be a feature in shopify 

All I want is an actual number of units sold. If I add 50 units of a product then another 10 and then 20 etc I want to quickly find out how many I have sold altogether.

Unfortunately it doesn't work for me as I've probably changed the inventory a few times. I just want to go to the products and it can tell me how many I have sold total of that product

You should not have to buy an app. It should be a standard in shopify as most businesses would want to know how many of a certain product they have sold.