(Note: the relative "who" and the interrogative "who" have rather different grammatical properties. When I talk about the word "who" in this answer, I'm specifically talking about the interrogative.)
In many languages, the subject of a verb has some effect on the form of that verb. For instance, in Latin, "the boy runs" would be puer currit, while "the boys run" would be puerī currunt. English still has some slight vestiges of this system by now: in the present tense of regular verbs and in all tenses of irregular verbs, the third person singular is marked differently from every other form.
So if you had an explicitly singular subject, as in "the old man lives there", you would use the -s on the verb. And if you had an explicitly plural subject, as in "the old men live there", you would not use the -s. In Latin, you would do the same with "who" on its own.
But in English, unlike in Latin, "who" on its own is always considered singular. You'd only use a plural form if something else in the sentence explicitly makes it plural. Otherwise, the singular is used.
So even if I knew there were multiple people involved, I (as a native English-speaker) would ask "who was at this church today?" rather than *"who were at this church today?". (The star means that this sentence is ungrammatical to me.) It would only take plural marking if some other word, explicitly plural, were added: "who were the parishioners at this church today?"
