By my calculation that's 18.5M Ohio State fans and 500K Michigan fans. The second largest audience just after the 2006 game which was a 3:30 game. If they played this game in prime time, it would easily draw in the low 20M's 

https://www.sportsmediawatch.com/college-football-tv-ratings/

Because I'm a nerd and retired with time on my hands. I averaged all of the top teams this year for viewership and Ohio State came in first with an average audience of 7.04M. Followed by Alabama with an average 6.39M and Michigan third with 6.01M.