"Do you have any letters?"
Translation:Har du noen brev?
So you will need noen to make sure "brev" is plural? otherwise it would be "Do you have a letter?" right?
Presumably "a letter" would be "et brev". Without "noen", the sentence would translate to "Do you have letters?", I think.
If you asked me "Har du brev?" I would immediately understand it as plural, or at least optional when it comes to number. Going to the post office asking this question, they would probably answer by asking how many you need. The word 'brev' is both singular and plural. Very often in Norwegian you wouldn't translate the word 'any'. At least we use the word 'noen' with this meaning less often than English does with 'any'.
It seems you can say either "Har du noen brev?" or "Har du noen brever?" - both ways are acceptable.
I checked the dictionary and it agrees with you - but as a L1 Norwegian speaker, I must say that "brever" sounds wrong to me, and if anything rather archaic.
Indeed, as is the case with all one-syllable neuter nouns, and many longer ones as well. :)
However, "brev" is the more common plural form - by quite a margin.
It's hard to translate the word "any" to Norwegian. In many cases I would actually prefer not to translate it at all, and let context show the meaning. For this sentence it seems to me that the translation "Har dere brev?" would be perfectly fine. I imagine myself in a shop asking whether they have any letters to be sold.