First, I think there is a limit to how many dogs a breeder should have. Many boast about their "breeding facility." What? No. Dogs belong in our home. The same goes for dogs who are having puppies, have had puppies and those puppies who are getting big. Yes, in the home; not in a facility built specifically for the mass production.
Some breeders have "breeder" dogs out living with families. The dog comes back to the breeder to have puppies and once the puppies are weaned and/or placed; the female goes back to living with her family. I like this arrangement if it is done properly. Few breeders keep studs around. I know many breeders who just don't want to deal with the boys so they go out to stud. Of course this is a good way to support genetic diversity. (If of course, the breeder supports this in their stud choices.)
There are just too many dogs out there. I love when breeders only breed when they have a waiting list. A litter can be partially or completely gone before it even arrives. This of course is the best way to do it. When a breeder just keeps breeding and breeding and breeding; they can end up with a lot of over flow. This can result in the money making breeder, letting go of their goods for less and with little concern for where they are going.
How many is too many? The question has many answers. Relating to adult dogs living in a home; one must look at what they can offer as care, nutrition and attention. Litters of puppies? That can be answered by a waiting list. For those breeders out there who are pumping puppies out, assembly line style. PLEASE STOP. The goal should be quality, not quantity.