As far as I know, there isn't anything legal about it. There are reputable breeders, and puppy mills/puppy farms. A reputable breeder will usually belong to a reputable breed club/registry and follow the code of ethics set forth by that club.
Each breeder choosed to breed how they wish. I personally breed, breed, skip a heat. I then spay my girls around 7yrs old give or take. Some breeders will breed then skip a heat, the breed, etc. A reputable breeder will also wait until proper health testing can be done on the dogs they are breeding.
The improper way to breed in my opinion, is every heat all the time until old age.
Illusionminis is correct, there is no legality issue unless they are a kennel or breeder and are licensed. There is a code of ethics, and ethical practices would not recommend breeding continuously.
3-4 litters is plenty for a bitch, but if someone is going to bring that many dogs in to this world, I would hope that the dog has an excellent, proven conformation, personality, is registered and has been health tested. Breeders should plan on spending about $1,000 and a lot of time and training to see if they have a good breed bitch, and if they personally possess the qualities to raise, educate new owners and support them through the life of a pup.
He is your friend, partner, defender, your dog. You are his life, love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion.