Yeah I think shows like NCIS, Mentalist, CSI, House etc get 12m+.
And the ratings fluctuate a lot due to big competition from 3 other channels. For e.g. I was looking at House's ratings from last season and one week it had 17.2m, the next week 14.4m and then within a couple of weeks dropped to 11.7m.
Even lower rating stuff like Modern Family fluctuated a lot: one week it had 7.2m, then 9.7m, then 10.2m, then back down to 7.8m! We don't see shows over here fluctuate anything like that unless there's special circumstances. The American ratings would be really hard to predict imo and I think there'd be loads of 0s. An interesting suggestion though.