There is no real plural in Japanese. "入ってくる娘が---" means "the girl(daughter) who will join" but could also mean "the girls who will join" as they have no idea of how many and can't simply put a "tachi" behind it as that would be wrong too. =_=
They have no idea, Tsunku probably still doesn't know, there is no set number. I'm prepared for more than one, since Tsunku's on a roll lately.
