You're confusing interrogative pronouns ('Wh--question words') with relative pronouns.
1. Question words ( Who? What? Where?):
You are right that 'Who' always needs a singular verb when it's an interrogative pronoun in a subject question. For example, you say "Who wants some ice cream?" or "Who is going to the party?". You would never say "Who want...?" or "Who are...?", even if you are talking about a number of people and you know that the answer is plural.
2. Relative pronouns: (that, which, who, whom..)
The above rule does not apply when 'who' is in the middle of the sentence, acting as a relative pronoun which connects two ideas and avoids repetition of a subject or object.
I know a person who lives in the centre of London = "who" refers to 'a person' and needs a singular verb
I know a lot of people who live in the centre of London = "who" refers to 'people' and needs a plural verb