Not sure, I would estimate 15 million. It was maybe 17-20 million a couple decades ago, and has been declining.
The population of midwest states totals nearly 68 million people. According to the Census Bureau 84% of the US population lives in suburban and urban areas. That gives an estimate of nearly 11 million people living in the rural midwest.
I was initially just snarking, since I ain't done seen no subways when I lived in Indiana, but instead I learned about population distribution.
Personally, I would have figured a majority would have been rural (since there's just so much rural land area). I hadn't realized it was so sparse.