I think your right in that it could be a plural, but 'jij' isn't the plural of 'je' but rather an emphasized form of 'je'. The plural form of 'you' would be 'jullie', so I guess an accepted answer should be: 'Horen jullie enkele koeien?'.
When I was asked to translate the sentence, Duo told me the correct answer was "Hoor jij een aantal koeien?" But "een aantal" means "a number of" so why was that accepted as a correct answer when it seems much more general than "een paar"