There are a number of things you need to know:
1) the forward voltage of your LEDs. This will usually be specified as something like Vf@20mA in the LED specs and may be a range of voltages. Take the typical value or mid-way between the min and max.
This will differ between different types of LEDs, but mostly between colours.
2) your supply voltage you said 12V or 24V, but it is wise to know the range of voltages which may be present. For example, a car battery may vary from 10V to 14V. Larger variation will make things harder and will also reduce the number of LEDs you can place in a string.
Then go
here
As your source voltage, enter your MINIMUM input voltage (say 10V).
As the diode forward voltage, enter the number you got from the specs.
As the diode forward current enter 20 (20 mA) or whatever current you want to run them from.
for the number of LEDs in the array, enter a large number, say 100.
Then click "design my array"
You will see a diagram showing a number of strings of LEDs and a resistor all connected in parallel. Note the number of LEDs in each string, and the value of the resistor.
Now go back to your input fields.
Change the input voltage to the maximum voltage your power supply will deliver.
Change the number of LEDs in the array to the number placed in each string previously.
Now hit "design my array" again.
Note the new value of the resistor -- this is the value you should use.
You will need to do this for each type of LED you have. Possibly only twice if you have red and blue LEDs which are all the same. Possibly more often if you have LEDs from a whole heap of different manufacturers.
The problem with using a single resistor like this is that the brightness will fall off quite rapidly with voltage. If your voltage source is stable (i.e. the min and max voltages are very close) then you will have fewer problems.
After you've done this you will have to wire up the LEDs in many strings and connect them all up.
I hope that's clearer.
Whether or not you get enough light at 20mA is another issue. Perhaps you should check the specs to see what the maximum continuous current is. It may be 25 or 30 mA (or even more) and you would be wise to use the max value as stated in the specifications.
If you don't have any specifications, you need to find another source of LEDs that can provide them. If you've already bought them, you could measure the Vf by estimating the Vf and calculating a resistor which will limit the current correctly, then measuring Vf to verify.
