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.
Asking about the subject:
Use third-person (singular) form of verb after 'who' even if you know the answer must be plural.
Who has a pen today?
Who is outside now?
According to Michael Swan "Practical English usage, Oxford 1995" when 'who' and 'what' are used to ask for the subject of a clause (as in this case), they most often have singular verbs, even if the question expects a plural answer.
The girls are playing outside. → Who is playing outside?
Asking about the Object:
Here you simply make your verb agree with the subject. Actually in this case the formula used to make an information question is exactly like other wh-words like where and when.
They meet him every Saturday.
Who do they meet every Saturday?
You see after 'who' we use 'do' here because 'they' is the subject here so the verb must agree with it.
(Note that if you want to ask a question about the subject, you'd say, "who meets him every Saturday?")
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.