Yes, neuter words usually don’t change in the plural, unless they end with a vowel.
Assume that a few boys are writing one letter.
"pojkarna skriver ett brev."?
I would argue that you would actually say "Pojkarna skriver brev" also in the case that the boys are indeed only writing one letter.
True, skriva brev can refer to the act of letter-writing, so it's really neutral in terms of how many letters are being written. If you want to be clear that they're writing just one letter though, you have to say ett brev.
This reminds me of the Spanish "escribe" and English "scribe"/"inscribe"