I don't understand this. My point would be like this:
As they have asked the number of keys as a whole and not summed up on individual knowledge of no of keys for each person, hence what I could think of is:
As per symmetric cryptography we can communicate with a person secretly if we have his public key, and in turn that person will use his own private key to decrypt the message and no one else.
So each person posses a Public Key & a Private Key each, and so for N persons with 2 keys each.
the total number of keys in the system would be 2*N = 2N to suffice a confidential communication.
To think of the whole system I would think like: I have a huge whiteboard on which N public keys are written and each N person possesses his own private key which is secret.
Please correct me if I am wrong somewhere.