Since "qui" is the subject in "Qui parle sans cesse?", it is singular.
It would be different if you said "Qui sont ces hommes qui parlent sans cesse?", in which case the subject would be "les hommes", which is plural.
If the verb is "être", then it will be in the plural: "Qui sont les parents?" or (to steal vc 74’s example) "Qui sont ces hommes qui parlent sans cesse?"
Otherwise, it will generally be in the singular.
English is exactly the same in this regard: "Who is making all that noise?" vs "Who are the people making all that noise?" (The "is" in the first sentence is just an auxiliary verb, used to construct the present continuous form "is working"; the "are" is a fully-fledged inflection of the verb "be".)
The interrogative word qui always triggers masculine singular agreement. You can see it as lacking number and gender features, and thus agreement defaults to the least marked number and gender: "Je vois que tu as eu peu de visiteurs, qui est venu ?" (even though you know there’s been several visitors, the verb is still singular)
The relative pronoun qui however, can trigger all sorts of agreement, because it carries its antecedent’s gender and number features to the relative clause: "je ne connaissais pas les femmes qui sont venues"
So you need to be able to recognise which qui you’re dealing with if you want your verb to behave correctly. Indirect questions like "je ne sais pas qui est venu" are a trap in this respect, since they look superficially similar to relative clauses, but aren’t.