How do people with 6+ kids survive?

I have three very kids under 5(10 month old who is crawling very well and getting into everything, 23 month old who aids the 10 month old in getting into everything lol, and a 4 year old) and I currently started babysitting for a family with 3 more! They have a 16 month old baby, a 5 year old with cerebal palsy, and an 8 year old with ADHD. I am constantly on the go! I feel like I am physically exausted to the point where it is hard to function. I first started babysitting because we were about $300 behind on monthly bills so we just needed a little extra money and I really enjoy kids. My husband is now working more than enough overtime, but he is afraid it wont last long. I enjoy taking care of these kids but this many abd on top of that some with special needs is very difficult. I am ready to just call it quits but I am afraid my husband will be mad/upset because I quit. What do you guys think I should do? How do people that actually have this many kids survive??