What's the purpose of this serial input?
its an oscillator in max/msp that inputs values 0-255 in a random sine wave to the arduino. so when the leds light up they pulse according to these values.
Does it send one set of data, or does it send multiple?
it sends one number, 0-255, constantly over and over.
How does your serial device (sending the data) knows how many LEDs are on?
it doesnt know how many leds are on.
Where things get complicated is when you have multiple LEDs going at the same time (when the button was pressed multiple times).
yes this is where i hit a wall.
sorry for the delay in response. the storm took my internet for a few days.