I tried to add 5 of the item "Flask or Tankard" to a character; I wanted to customize one as a flask and the other four as tankards. I added 1 of the item, and customized it as a flask. I then added 4 of the item, and customized them as tankards. What I was left with was 5x of the item, customized as tankards. I finally compromised by noting the subcounts in the description, but this is suboptimal. I've seen it work with wieldable items, which seem to each get their own entry (e.g. an Artificer's infused armor) but perhaps it doesn't work with commodity items?

Separately, for pure flavor's sake it might be worth making flasks and tankards their own items from the get-go. But that's just my opinion.