It isn't numbers but rather quality and vision. Someone can breed 1 mare a year or 20 mares a year, but what makes them a breeder versus a backyard breeder is the quality of their broodmares, the care of their horses, the breeding decisions they make, the follow through with their young horses, and ultimately, the end result.
This isn't the first time the whole "what is a backyard
breeder" conversation has appeared here. Unless you are trying to define a tax status, I don't understand why the label is important. You are either a good horseman, or you aren't. You are either ethical, or you aren't. I probably know more people with an inflated sense of their abilities than I know that don't. It doesn't make me scratch my head... that's just life.
I have 15 acres, and two (now retired) broodmares. I made some good decisions when breeding, and I made some mistakes. Who ever got everything right? I don't particularly care whether someone labels me a backyard breeder or not. The only legacy that has been important to me is being considered clear-eyed and ethical.