Is the word "who" singular or plural. For example,
Who have come or who has come? Or is it both plural and singular. As 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"