Iowa as the state that has the most? This is just based off of the assumption that the reason you think that this might be something people may know is the book and then film The Bridges of Madison County. That being in Iowa. Also birthplace of John Wayne (the real "Duke" not John Wayne Gacy

) As far as the number of bridges I have no clue. I never really read the book or saw the film.
ETA
I meant to say that I thought Iowa was the state with the county (Madison County) having the most covered bridges. Still no idea on the number. My guess for the state with the most would be Pennsylvania.