Have is both singular and plural. 'I have' refers to one person, 'I', so it is singular; yet 'We Have' refers to multiple people,'We', so it must be plural.
A singular subject has a singular verb form. He/ she/ it/ and singular noun subjects like the boy/ my brother are singular subject and the verb form for singular subjects is verb + s He likes ice...
The interrogative pronoun 'who' is both singular and plural; for example: Who is the winner? or Who are the winners? The relative pronoun 'who' is both singular and plural also: The Browns, who are...