***Neither a teacher nor a native speaker.***
Whose is correct for both sentences (plural and singular).
Here you can see the differences between whose, who and whom.
This is the girl whose car was stolen.
-> Whose car was stolen?
This is the girl who stole my car.
-> Who stole my car?
This is the girl whom I stole the car.
-> Whom did I steal the car?
Sometimes whom can be replaced by who.
Student or Learner