Is one list enough, or do I need one per supermarket?

One list is enough in almost every case — and that's entirely by design. In pon, several supermarkets live on the same list; it's grouped by store automatically, and while shopping you only see the section for the store you're currently in. Creating a separate list per supermarket works against this idea and gains you nothing — we'd advise against it.

Multiple lists only make sense when you genuinely have separate purposes, like a „Weekend“ or „Party“ list alongside your regular shopping. For everyday use: stick with one list — it can do it all.