I'd suggest starting with 1, get the resistance values figured out for your particular RGB LED, get some basic code to work to understand the datasequence needed, then build up from there.
To multiplex, you will need banks of PNP transistors or P-channel MOSFETs to control the voltage to the parallel'ed LEDs.
What kind of architeture were you thinking? 'x' banks of 'y' LEDs using 'z' WS2803s?