The answer is 285 ohms for 9 volts or 435 ohms for 12 volts. More important is how I got those numbers. I took the supply voltage (9), subtracted the running voltage (3.3), and divided by the current (0.020 A) used by the LED for 285 ohms. If you substituted 30ma (0.030 A), you would get 190 ohms. Since these are odd values, you might like 220 ohms (in the middle) or 330 ohms (my first choice). You will find that the blue LEDs are prety bright either way- oh yea, they are neat too.
Question #2. It depends. Are you putting them in series or parallel? Either way, recalculate the voltage and current changes based on your configuration and use the method for resistor calculation above. You know. For instance, if you put them in series, the voltage goes to 6.6 etc.
Enjoy.