According to this article there were 50,000 people (in 20,000 households) whose TV viewings are counted towards ratings:
http://splitsider.com/2011/01/why-nielsen-ratings-are-inaccurate-and-why-theyll-stay-that-way/
This article mentions that Nielsen viewers live in 31 “meter markets”
http://tvline.com/2014/05/29/tv-ratings-nielsen-to-increase-sample-size/
It looks like larger, liberal areas may be oversampled based not this data. So it’s unclear how many people living in suburban and rural areas are being counted.
Actually, Nielsen—thanks to cheap computer technology—is now seriously looking at increasing its sample size by a factor of ten or more. Essentially, year-round “sweeps month” measurements with over 100,000 metered households.