The breeder described by the op could only have one or two breeding bitches at a time. Each only having two or three litters.
Bitch 1: litter 1 at 3 Select bitch 2.
(18mo later): Litter 2 at 4.5
(18mo later)
Bitch 2: litter 1 at 3 (select next bitch)
(18mo later): Litter 2 at 4.5
(18mo later)
Bitch 3: litter 1 at 3 (select next bitch)
And so on. Assuming they live 10-12 years you'd have maybe 4 bitches alive at a time,
Quite often breeders have their bitches out on breeding contracts in pet homes, so they have more litters because the bitches don't live with them and only come back to mate and whelp. If you breed much less you'd struggle to keep the lines going and still see grown dogs so you can 'select' the best, and cover yourself if you have a bitch who doesn't take to motherhood so you maybe don't want to use her again. At that point you're breeding from your family pet instead which is also a mn no!
We had one litter from ours, with a view to it being an ongoing thing, did all the research, registered with the KC, picked the best stud, etc, we kept a bitch. The first one really didn't take to it so we had her spayed.
Circumstances didn't work out so we had the second bitch spayed when it became clear it wasn't going to be a thing we could do properly, but I'm not sure what mn expects from breeders tbh. You have to find a secret breeder who doesn't advertise puppies and only breeds once a decade or something, but also isn't just having a litter from their pet.