Go to attributes settings

Store -> Attributes -> Products

Open attribute with the label "Quantity"

Open tab: "Storefront Properties"

Set "Used for Sorting in Product Listing" to "Yes" and save it

Open category where you want your sorting.

Catalog -> Categories

Open your category, now open "Display settings" tab and set "Quantity" in "Default Product Listing Sort By" dropdown

Now to add descending sorting, open tab "Design" and add this xml code here

<referenceContainer name="content">
<referenceBlock name="product_list_toolbar">
<action method="setDefaultDirection">
<argument name="dir" xsi:type="string">desc</argument>
</action>
</referenceBlock>
</referenceContainer>

Refresh frontend category page and see if it is working