In a question like this one, for example,
Who have/has come?
Is the word "who" singular or plural? Or is it both plural and singular?
I have heard it’s plural. Please correct me if I am wrong.
Who can be either an interrogative pronoun ("Who is that?") or a relative pronoun ("The man who sells fruit"). Neither interrogative pronouns (question words) nor relative pronouns (which/that/who and variations) are bound to grammatical number by themselves. The plurality is instead bound to the object in question.
"Who is that man?" - singular due to "man"
"Who are those people?" - plural due to "people"
"The man, who is sitting there,..." - again singular due to "man"
"The men, who are sitting there,..." - plural due to "men"
There are two possibilities for 'Who' as an interrogative pronoun: it can ask about the subject or the object of the sentence.
About your question, based on what has been mentioned, "who has come?" is the preferred choice.
Please note that I wrote just about 'who' as an interrogative pronoun and not as a relative one. 'Who' in a relative clause agrees with the noun it modifies. It can be plural or singular.