For me, 2-3 would be maximum. The thought of never having a moments peace would fill me with horror. I also couldn't be bothered with the whole cooking, bed making, laundry for loads of children.
2-3 means with two parents, they can tag team easily, or one can take then all easily somewhere. They also can fit in a regular car, and grandparents potentially offer to babysit occasionally.
20+ every day is how my busiest christmas day or a garden party would be, and its heaven when they all go home.