Ok, so I was just wondering if running a 12 volt power supply through a 19 ohm resistor is practical for lighting 32 LEDs (in parallel). Like, I know you can do it, but I never see anyone use resistors that are that small. If it's not practical, is there a certain number of leds/minimum resistor value you guys could give me. thanks.
Hi,
Something sounds wrong here.
Why would you put 32 LEDs in Parallel?
12 Volt systems that power LEDs usually put groups of LEDs in series, with an appropriate resistor.
First figure out how to do one string of series LEDs, and what resistor is needed.
Then figure how many strings you need.
Then juggle the number of LEDs in each 12V string so you can have about the same number of LEDs in each string.
Find the forward voltage drop of the LEDs you have at the current you want. Maybe that is 2.1V if they are Red, higher if other colors.
Example: 2.1V Red LED: 2.1 * 32 = 67.2 TOO high for 12V, So divide into strings:
Maybe string of 5 LEDs? 2.1 * 5 = 10.5V and then a series resistor. R=E/I MAYBE 12-10.5=1.5V drop across resistor.
IF 20 mA is bright enough: R=E/I = 1.5 / .02 = 75 ohms. Standard value 68 ohms is fine.
6 strings of 5 LEDs is 30 total. OK? Or juggle more.
.. get out your calculator 
The problem with LEDs in parallel is that if you want equal brightness they all have to have identical characteristics. There are production variations, and you're operating the LEDs at the knee where a small change in voltage results in a large change in current & brightness.
It might work, but it's bad engineering practice.
And, you could potentially end-up with most of the current going through 1 or 2 LEDs, burning them out. If the burned-up LED happens to end-up shorted, none of the LEDs will light, and since they are in parallel you won't know which one is blown. Or, if LEDs start failing-open the current will be concentrated in the remaining LEDs and they might begin to fail.
There's no "minimum" resistance but 10V across 19 Ohms is about 5 Watts. And, it's a good idea to derate resistors, so you'd want to to use a 10W resistor.