
I think a lot depends on the breed and size of the litters. I have has two litters from my oldest bitch (only 8 pups altogether) and have had 3 from her daughter and Grandaughter (both had a total of 17 pups).
I like 3 litters as this gives you a chance to try two different mates for her to see which produces better results, and gives the option of trying a third, or repeating whichever of the first two gave best results.
I cannot see why anytone would need to have more than four litters from a bitch to find something worthy of keeping from her unless it was in a breed where only ones and two's are born and you could wait ages for the sex and quality you were looking for.
If you are trying to breed for improvement in your individual bitch line I do fel a bitch needs to have at least two litters to two different sirtes in order that you can learn which traits she passes to her puppies herself.