The only available ratings are 0, 1, 2, and 3.
Presumably they mean he has been awarded 3-star standard by Michelin and has 6 other times in the past (or some other combination adding to 18 prior stars). Of course one could equally mean he's been a 1-star chef for 21 years which wouldn't be quite so impressive I feel.
Weird, I always thought, the way it's presented on British TV shows, that the Chef de Cuisine was rated but it makes more sense to rate the restaurant as an ensemble. I suppose then though he doesn’t hold the stars the restaurants do; and he can't really be head chef at each if he's not there most of the week.
Interesting though, thank you for the correction (and to the sibling commenters too, thanks).