How fun my first comment, and just to post an exception here. I would use pieces to refer to potatoes if there are pieces of potato here. For example, if someone offers me a serving of breakfast potatoes with the question "How much do you want?" or "Care for some potato(es)?" I could reply "Just a few pieces of potato please, only potato." in order to emphasize that I want just potato pieces on the plate and none of the bell peppers, onions, or whatever else may be in the dish. :)
Thank you, I definitely don't know everything, but I do know Russian, and the Germanic loan words tend to stand out for a Swedish learner. :) – I just googled stycken and it turns out the early Germanic meaning of the word which both the Swedish and German word come from was a diminutive for the word stock, which means approximately log in English and бревно in Russian, so it all comes from a small unit of timber, isn't that fascinating?
It translates into english as "Two, please" (or "Two tickets"). Ie, you don't translate "stycken" literally most of the time. You can translate it as "Stück" to German 1:1 though.
I think the long explanation is, in english, the "two" automatically means "two of whatever you give everybody else", ie "two tickets", "two glasses", etc. In Swedish you use "Stycken" for that, in German you use "Stück" -- ie, "tva stycken" / "Zwei Stück" means "two items", "two instances of whatever it is you give". Depending on the context, it sounds a little strange if you don't use "stycken" or some noun after the number.
I hope that doesn't confuse you even more ^^