It can be either plural or singular, Adding ~들 only clarifies plurality. Using the topic particle ~은/는 indicates what's being talked about (strawberries or a strawberry) so this sentence is actually a general statement about strawberries.
Interestingly in English plurality isn't needed either for generalizations. To say "cats are animals" one could also say "a cat is an animal" so to say "strawberries are expensive" one can also say "a strawberry is expensive". Hopefully this clarifies why clarifying plurality wouldn't be needed when making general statements.
So the "달기는" is plural?? How to know that the word is plural or no besides using "들" ?
From the little bit of Korean I've picked up it seems to me that many things in English that we need to differentiate between singular and plural don't apply nearly as often in Korean unless it is needed. Kind of like sayng "look at that paper." If the person points to a stack of papers you would know they are referring to all of the papers. I have a hard time with this concept a lot, just like I have a hard time doing of the subject of the sentence in Korean too.