I believe that Kiyoshi's parents had so many children due to there culture, I dont think that it was planned when his mother would get pregnant, just like he says in the book, she did not know anything about rubbers. I think that she knew that she could possibly get pregnant but, I think that her thought process was if it happens its meant to be that way.
I also think that they had a lot of children for the future. I think that when people think of having children, they think of raising them, they think of them small, but I think this family, and many others in the world, think of the future, then they are old they are going to have a bunch of children to take of them, and I believe that they also think that not all of them are going to turn out to be just how they want them to be, so if you have many children some of them are bound to take care of you when you age. I think that if they were to of had all girls, they would of kept trying until they had a boy, its just culture.
Another reason could be that when Kiyoshi's parents age, they know that there children will help pay off there debt that they have, they will be able to work and help out when needed; and eventually take out there household and take care of them sense they cared for them as children. I think that it depends on the families, and past and present, today I think that people would love to have more children but with the cost of living, its just not possible if you want to give them everything that you want to give. But then there are still families today that are in the same mind set from the olden days, and do not want to be lonely when they are old. In my family I have 3 sisters, I am not sure why my parents didn't go for the boy but I am thinking that they had the thought process of raising 3 girls, not that we would care for them when they are elderly and sick. As for my husband's mother, she had 6 children, and says that she would of had more if she could have, her reasoning for having so many children is not wanting to be lonely when she is old, and not wanting to have an empty house on holidays. To to each is own really but that is my theory as to of why Kiyoshi's parents had many children.