Most reproductive vets are now suggesting that a bitch should be bred back to back, as it is healthier for their uterus that way as long as they are otherwise healthy. While most breeders are now aware of that, it is still hard for many to do. I don't know of anyone who has done more than 3 back to back.
With Trouble, I have done back to back breedings, but she doesn't always come in regularly. This current litter is due October 19, her last litter was born Oct. 3, and the litter before that, December 21.
I do agree that most breeders have between 3 to 5 litters from a bitch, depending on the health of the bitch and the quality of her litters.