Good question. I think that a puppy mill's description varies by the individual. IMO, someone that has 15+ breeding bitches that live in an outside kennel would be a start. Someone that does just about ALL of their veternarian care would also be in this classification. Someone who's dogs are not groomed and cared for. Somone that has large amounts and several breeds and also advertise that they take credit cards.
No one really knows until they go see the place as you stated. It could be a small scale puppy mill or a large. Everyone's definition will be different.
When we went to buy a female GSP, once...there were probably a dozen breeding females. Males were kept in a cattle pen. Bitches were chained to trees with puppies running everywhere. To me, that was a puppy mill.
But I recently heard of a show breeder that has over 40 breeding females.