21 spline is adequate for the majority of people.
I'm not sure if the splines themselves are stronger of if it is due to the larger diameter of the shaft required to "fit" two additional teeth on. I believe I have seen the shafts sheared off which makes me think it's the larger overall diameter. If this is the case,
Think of it as a carrot that you want to twist into two pieces. A smaller carrot will break easier because it has less area resisting the twisting force you put on it.
Tell me if I'm patronizing you, that isn't my intent, just trying to compare it to something you can visualize or have experienced.
