Zero would be plenty.

The question assumes it's not possible for a person (or more likely, a man, his wife, and a handful of children - perhaps along with his brother or cousin and family) to live independently. In reality, the sort of person who would purchase Midwestern land and settle there (and, well, not die immediately) would be someone with enough skill in farming, hunting, carpentry, metalworking, etc. to build all the structures necessary and construct/repair all sorts of tools.

Such a person is not trying to "start a township." There's no money in that! The money is in the land, and what products can be made of the land. A "company town" really did not and could not exist prior to the Industrial Revolution in the USA - skilled workers are very useful but without industrial machinery the scale of production isn't enough to require trade specialization.

So let's think of the settler as a businessman. He has the skills (or brought along close kin with the skills) to clear land, build a home, make and maintain a farm, and sell something to some distant but accessible market. If, perhaps, he chose land very strategically - a good location for a mill - he might develop an anchor point for a town. This is what tends to turn a single-family settlement into a community: build a proper sawmill or gristmill and you no longer have to travel anywhere; the loggers and the farmers come to you. That's when supporting businesses - grocers, carpenters, blacksmiths, etc. - start to set up shop.