"I am not a number - I am a free man."
Well that might be all fine and dandy for Number 6, and I suppose there are many who would admire such a stance on identity and individualism. But when push comes to shove, there was a time when even Number 6 positively fought to maintain his identify as Number 6, in his determination to prove that he is Number 6!
Having been stripped of his identity of the outside world, and being turned into nothing more than a number. The Prisoner became afraid of losing his identity of No.6 to another, and fought to maintain even this identity to prove who is.
But they know who he is. We know who he is. Only Number6 has doubts about who he is!