Mike: The difference in brightness is based on how many milliamps you run through the LED. The 20ma and 10ma values are just in the ballpark values. You should not go higher than 20ma though. Any more will tend to shorten the life of the LED. You can also go lower than 10ma which will dim the LED further. But I wouldn't run it at less than 5ma.
So anywhere from 5ma to 20ma can be run through the calculation for the resistor value to give you the brightness level that you desire.
The difference in wattage values has to do with the amount of current in the circuit. The resistor needs to be hefty enough to carry the load. Higher current, higher wattage and the resistor should be rated to handle it without generating a lot of heat. The LED calculators on the Internet will tell you what the wattage should be.
As far as the number of LEDs that you can run off of your 12vdc power supply, again it depends on how much current you want to use with each LED. If you use 20ma and you hook up 100 LEDs, you need 20 x 100 = 2000ma = 2A. Your power supply will be rated to be able to put out so many Amps. So a 1A power supply could run 1000ma / 20ma = 50 LEDs.
