I am not arguing any number is a standard rate.
I have checked and on Masterchef alone they start with 58 so over 20 years 4,000 is a believable figure especially when you also add all contestants on Master Chef Professionals, so again can you answer one question do you not find it strange only 13 of all the the people he has worked with over 20 years have come forward now, surely if telling dirty jokes and being a bit pervy was the norm and he saw no wrong in doing it he would have told these jokes in front of all contestants not just these 13!
I think you're making the assumption that because only 13 made a complaint, that only 13 found his behaviour objectionable. Given the amount of coverage the story has gotten, I can't blame people for not coming forward.