Some words come only in plural. Here’s a list of some of them in English. This phenomenon is called plurale tantum. For example, “scissors” or “jeans”. In Polish it’s the same. There’s no door but rather “doors” — drzwi. Same with “birthday”, urodziny, or with holidays, ferie, a phenomenon that also appears in German (ferien), or skrzypce, “violin”. There’s many examples, can you come up with some?
What happens when you want to say “two doors”? You can say that — you just have to choose the correct “two” to use. In these cases, it will be the collective two: dwoje drzwi, dwoje skrzypiec (with the genitive, because the collective form always takes genitive).
There’s also singulare tantum, for terms only appearing in singular. Like “information”, “news” or “milk”. Often these are uncountable nouns. A typical mistake among Spanish-speaking people is to use “news” as a countable noun, “I have two news” — that’s because in Spanish, noticia is countable. In Polish, wiadamość is also countable.
Sometimes a word can be used both in both singular or plural, but then the meaning is different. In Spanish (at least in some regions) the plural lentes means “glasses”, but the singular lente means lens. Conversely, you typically say gente, “people”, but you can say gentes, “peoples” in some specific contexts — similarly (but not quite) as in English. (For another fun example to compare English/Spanish, consider “water(s)”/agua(s)). In Polish, glasses (lentes) are okulary, which are also plurale tantum. A lens (lente) is soczewka — which has appeared in this blog before.
We see that sometimes the plurale/singulare tantum carries across European languages, but sometimes not quite. Here’s a phenomenon that doesn’t happen in Spanish or English because of the lack of cases. If I want to decline a word, I need to know its gender, which in the case of plural it’s divided between “male” and “not-male” (the only distinction in plural). It’s often not obvious. You just have to know it. In the case of names of locations which occur in plural, it can even confuse native speakers. Should I say człowiek z Borków or człowiek z Borek, to mean “a person from Borki”? (It’s Borków.) I got this example of a book I got not long ago and of which I’m very proud: Etymological dictionary of cities and boroughs in the People’s Republic of Poland.