The panel of celebrity judges just auditioned a selection of the applicants picked out by the show's producers to make good tv. So he did in fact audition all the ones you see on tv, but not all 50,000.
Well he is really a busy gentleman! He's got money to make elsewhere and I always wondered that if he really was so busy why did sit through hours and hours of "boring" auditions.
I think that not all of the 50000 people actually would have got to audition, As they simply wouldnt of fit them all in the time they had. I would say half of them wouldnt of even got the chance!