There is a post about creating plurals in Polish: https://www.duolingo.com/comment/12354055
You can also check most popular types of declension - there are also plural nouns:
Here's the chart with endings. Unfortunately, there are many endigs for every case, so you would need to check dictionary to be sure. A Polish person with decent vocabulary would be able to decline many new words oneself, but this requires some experience.
This question is a bit tricky.
First, you need to know which grammatical case is used by the sentence (and you can know that from the context). „Kanapki” can be a singular genitive (thus referring to one sandwich) or a plural nominative or plural accusative.
In general, plural is used when referring to more than one object. But in Polish, if you specify the exact number of objects, you need to be mindful of which case to use. If you don't specify the number of objects or that number ends with digits 2, 3 or 4 (except when it ends with 12, 13 or 14), you use your cases normally. But if that number ends with any other digit (0 and 5-9) or it ends with 12, 13 or 14, every time you would normally use nominative or accusative, you use genitive instead.
That's why from the perspective of a programmer dealing with localisation Polish appears as if it had two different plurals, but it's actually a case shift.
So in other words, depending on the situation, „kanapki” can be used to refer to any number of sandwiches, but sometimes only when you don't tell exactly how many.