Thing is, I don't think you can possibly give the quality time a child needs to that many children, it just isn't possible. But I guess that's her decision. I wonder if there is an 18th child syndrome?
Keep in mind that this was the norm a 100-150 years ago - if the woman lived through it. Read the biography of Charles Wesley's mother. She was organized and schooled her children (don't remember how many but a lot), spent time individually with each one daily, devout to God, devoted to husband. I don't know if she had any servants or not. I am assuming she had some - but then we have washers and driers today.
I'm not advocating large families, though we had 5 children, but since it is not the norm people tend to shake their heads in wonder. At least they are Christian and he is not a polygomist. As for being tax exempt, I don't know if they have incorporated their family or not, but I have heard of it being done (prior to the Duggars). I assume they have developed a sense of "ministry" and therefore become tax exempt. The incorporation status gives them some sort of benefits as well.
No, my husband and I were not incorporated. We were simply in the poor house for many years.