What is Polyphony?
Polyphony is the amount of notes that will sound simultaneous on a keyboard.
Some synthesizers are ‘monophonic’ which means that only one note will sound at once and if you played two together, only the highest one would sound.
Most modern digital keyboards are polyphonic to 64 notes or more -
Considering that you only have ten fingers (presumably), you may think that any polyphony over ten would be fine. But when you consider adding multiple sounds together and using the sustain pedal, you will find that his will soon become inadequate.
Most quality keyboards have a polyphony of 120 or more.