in Spanish you can use the word "who" in plural if the subject refers to more than 1 person...so for example "quienes son mis amigos?" - who are my friends? (friends are plural) does this exist in Portuguese or is "quem" always singular?
"Quem" doesn't change, you can know if is singular or plural by the "é" or the "são". "Quem é o advogado?" = "who is the lawyer?" / "Quem são os escritores" = "who are the writers?"