
I think individual bitches and also their age and general fitness play a big part.
My Kizi at two had a litter of 9 and she was fit in body (not in coat) byt the time her pups were 10 weeks, and if she were a smooth coated breed I could have shown her then.
After her third litter at 6 1/2 years old (only 4) she was again in good shape by the time pups were 12 weeks, when I had her spayed.
Now spaying seems to have altrered her shape a bit and she is sort of billowy, though not overweight, but looks it to how slim she used to be, and I ahve cut her food down by a third to what she used to eat before spaying.
Sometimes in caoted breeds the coat grows back wrong after a litter (with mine kinks and waves, or off colour) and you then have to waith another 6 months for the coat to regrow when it comes right again. So campaigning a bitch in a coated breed is a nightmae if you wish to have them reproduce.