The library does keep track of how many pixels (or leds) you have with the statement:
#define stripSize 64
It then initializes the interrupt to use this in this statement:
LPD8806 strip = LPD8806(stripSize);
If you want to get into the nitty gritty, look at the LPD8806.cpp file. It breaks up the data to send to the ICs and then latches it in to display. ICs in series have information overflow from one IC to the next on the data and clock pins to allow these long runs.
Hope this helps a bit...