To answer your question, 'Polyphony' is a term used to refer to the number of tones that can be generated simultaneously before other tones start cutting out.
The CA51's 96 note polyphony is a reasonably good specification. I believe the more modern digital piano instruments from Roland and Yamaha offer 128 note polyphony, while as you are aware, some KAWAI instruments feature 192 note polyphony as standard.
It's important to remember that polyphony is not necessarily the number of keys that can be played simultaneously - after all, a piano only has 88 keys, thus one might think that 88 note polyphony would be sufficient for any performance.
However, this is not true. First, when playing stereo piano sounds, the polyphony is immediately halved. In addition, many instruments offer 'Dual' and 'Split' modes, allowing two or more sounds to be layered together, or the keyboard to be divided in two. The damper/sostenuto pedal also affects polyphony significantly, as keys are sustained while other are pressed. Finally, there are often additional effects applied to improve the authenticity of the sound, such as damper resonance, string resonance, and key-off samples.
As with most specifications quoted by manufacturers, polyphony is really just a number. While it is certainly important, I would personally be more concerned about the keyboard touch, piano tone, quality/power of speakers, and additional functions that add value to an instrument. In this respect, there is quite a lot more to distinguish the CA51 and CA71 than just the polyphony specification.
Now, without wishing to complicate matters, if you appreciate the touch of the CA51 and CA71, and are looking for an instrument intended solely for piano playing, I would strongly recommend that you also consider the CA18. http://www.kawai.de/ca18_en.htm
While the number of sounds and other features have been considerably reduced compared to the CA51, the polyphony and wooden key action are identical. In addition, the CA18 features an improved speaker system, more attractive, compact cabinet, and most importantly, the new 88-key piano sampling process.
I hope this information is useful.