I had a visit from a couple who have a great granddaughter of Tilly. They were telling me about their lab's dam and that she had been bred a number of times back to back. This has always been a controversial subject among breeders--how many times to breed and whether to breed back to back.
I would not consider breeding a bitch of mine unless she had her final hip and elbow clearances. And I have always thought that skipping a heat, especially if the bitch had a large litter or a C-section was good practice. However, there are reproductive specialists who indicate that doing back to back breedings is better for the bitch and the "production" of puppies. The contention is that the number of eggs produced declines with age and that the younger and more fit a bitch is, the better the whelping will go.
As far as number of litters, I would think that four is a plenty for a lifetime. All of my girls except Tilly (who had four litters) have had three or less litters. I just don't see the point in breeding them again and again.
Just like in anything that we do, there is my opinion and then there are others who have a different view.