"Drzwi" is pluralia tantum. It doesn't have a singular version, even if you literally mention "one door" it will be "jedne drzwi" - yeah, we just made the numeral 'one' into plural.
Then, do you depend on the ending of the adjectives to determine if it is just one door (the door) or more than one doors?
no. adjectives have to match the noun so they are already plural even if there is only one door. 99% of sentences number of doors is left amibigous. only if you actually add numeral/ quantifier you can determine how many doors are there
That's very interesting. Thank you so much for your input. I wish Polish Duo had some sort of comments/instructions per each topic instead of letting all the learners swim in the deep ocean by themselves !
Generally the same, but if the difference between 'locked' and 'closed' is really important to you, you can add "na klucz".