See the parent task, COMMERCE-3919, for context
- There are a couple different ways to do this, so you may want to look at Broadleaf and Magento 2 demos for examples
- OOTB, Magento shows a summary box where it shows how many units of each item are in the cart.
- I believe other platforms may display the quantity next to the option value name
- Also remember that, in the future, an option can be set as having a "user defined" quantity (buyer can determine how many units they want to purchase. See this mock). Different options may have different settings, so both ways of displaying quantity could be shown on the same product detail page at the same time.