Using Clay Travis math, this is the potential population of people in the Big 12 footprint. Considering that WVU has a sizeable fan base in DC and Pittsburgh, I threw all of DC and Pitt into the viewing area. Louisville is on the Indiana border too, so I'm sure there are Louisville fans there too. Again, using Clay Travis math/logic.
West
Texas 25,145,561
Oklahoma 3,751,351
Utah 2,763,885
Total 31,660,797
East
Kansas 2,853,118
Iowa 3,046,355
Missouri 5,988,927
Ohio 11,536,504
Kentucky 4,339,367
West Virginia 1,852,994
Indiana 6,483,802
Washington DC 601,723
Pennsylvania 12,702,379
Total 49,405,169
81,065,966
However, even if you take out border states and just add up population in those states, it gives us 60,000,000 people, so that's something to take into consideration.