8 weeks is the law in many states.
I think 12 weeks is fine if the breeder is doing plenty of socialization with the puppy. If the pup will be in the back yard, basement, or garage for another 4 weeks, it is preferable for the dog to be with you beyond 8 weeks.
Prior to eight weeks the pups miss out on some dog-dog relations that they get from their littermates, who are really the only age dog they should be with, with the exception of a good dam. The dam can be totally separate by six weeks or allowed to be with them.
Problems can be lack of understanding other dogs' body language and bite inhibition.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC
Hepzibah & Hannah