Translation:I want to forward this to my friends.
You are right. This is a sentence which leaves the plurality undefined, either the speaker thinks it is not important for the listener (no difference for him that the message is forwarded to 1 friend or to many friends), or the speaker just does not want to specify it.
There are certainly ways to express different situations in Chinese if the speaker wants to.
If you don't need this book, give it to a friend./如果你不需要这本书，送给朋友吧。
Here only 1 friend can get the book (as there is only 1 book), but which friend gets it is probably not defined yet.
I would like to give this book to a friend/我想把这本书送给一个朋友。
Here also only 1 friend can get the book. We can understand somehow from the context that the friend is actually already defined, although the listener does not know his/her exact identity.