I have several catalog items on the portal that are now showing the Quantity field. After searching around for a while I came across a HI KB article that addresses this. Hope this helps.

HI KB Article to fix Quantity field on Service Portal

In short, you will run a script (I ran it in scripts - background) from this article and it's supposed to run business rules on catalog items and is supposed to fix the catalog items to update the no quantity set to true in the portal settings fields -- and add changes to the current update set.

the script calls the migrate method in the CatalogPortalSettingsMigration script include. That script include's description states:

After Madrid, Catalog Item has portal settings. The script will help to migrate the following Item level settings for use_sc_layout=false to portal settings

No cart -> Hide 'Add to Cart'

No quantity -> Hide Quantity

Usage: var standardUpdate = true; // true: Call business rules on catalog item table and send changes to updateset.//false[DEFAULT]: Business rules on catalog item table will be skipped and no updateset changesvar psMigration = new CatalogPortalSettingsMigration();

psMigration.migrate(standardUpdate);