"The people talk to the king."
Translation:O povo fala com o rei.
You say povos when you are talking about two or more groups of people at the same time. E.g. Os povos argentino e brasileiro são rivais no futebol.
Note that "brasileiro" e "argentino" are both singular, meaning that there is only one "povo brasileiro", and only one "povo argentino" but "povos" in the sentence is plural because it referes to both of them.