Welcome to CPF, trigger1970!
:duh2: You meant 20 mA (0.02 ampere) ? That would be the common 5mm. diameter LEDs?
If so, total power requirements are low enough to not bother with a DC-DC converter to keep currents low.
That is: minimum voltage you'll expect to see from a 12V nominal lead acid battery? Or 3x Li-ion in series?
Anyway, if that's a battery then you have a voltage
range, not a fixed voltage like 11.1V. A general rule would be to put as many LEDs in series as voltage allows, while keeping some % of the total voltage left over, such that with a series resistor you'll have
somewhat constant current over the battery's voltage range.
In this case, 4 LEDs in series would be too many, but 3 LEDs in series would work. Series resistor is calculated using highest voltage, and allowable LED current. For example, if power source is 3x Li-ion (4.2V max per cell), allowable LED current is 20 mA, and LED voltage at that current is 3.0V, then you'd have:
R = (3*4.2V - 3*3.0V) / 0.02A = 180 Ohm. Hey that's a commonly available value! (E12 series).
P
resistor = (12.6V - 9V)^2 / 180 Ohm = 0.07W so regular 1/4W size resistors would do. It doesn't matter where in a 3-series LED string you insert that resistor, as long as it's in series. But watch polarity for the LEDs!
If LED or battery voltage is different, adjust resistor value as desired. Then you simply put X number of [resistor + 3x LED] in parallel. In this case, on each side 25 x [resistor + 3x LED] in parallel. So you need 50x those 180 Ohm 1/4W resistors, and a lot of wiring. But currents are modest (1.0A total @ 12.6V max), so don't worry about wire thickness other than as required for mechanical strength.
Fwiw: some length of LED strip would be far easier than wiring this # of individual LEDs...
