How many people could be trained to write a sentence in the exact same handwriting?

Not many I bet. Especially if you're trying to get adults to do it. And how much time are they getting to be taught? They would just have too much muscle memory to try to overcome in a short period of time.

The way I've heard it described is handwriting is almost like fingerprints in that every person has their own style and characteristics in each stroke. Pressure they put on the tip comes into play as well. Nobody writes the same even though we were all taught the same letters and basic strokes of those letters.