Some other factors to consider, in addition to the weight of the coaches as Alan mentions, are how well the train accelerates, and what grades are involved. I'd be surprised if a single AEM7 can't haul 24 Amfleet coaches at 100 MPH on track with a 0% grade, given enough time to accelerate up to that 100 MPH.
An AEM-7 would have difficulty supplying HEP to 24 coaches though,one would think.
Yes indeed acceleration and gradient are two big factors. All the horsepower in the world is of no use if it cannot be converted reliably into tractive effort.
IR took the trouble to design their own WAP7 after playing around with Bombardier's WAP5 and WAG5, precisely because the Bo-Bo WAG5 simply could not hack it on gradient. I believe they required certain acceleration capabilities on 2% gradient. Apparently the brawnier Co-Co WAP7 manages to pull it off with the usual 18 to 24 car trains on 2% gradients.
I have seen a single WAP7 accelerate an 18 car Rajdhani Express from dead stop at a signal up the gradient between Gaya Jct. and Koderma, quite effortlessly and been quite impressed with its pulling power.