Well, fairly simples.

The range of resistors commercially available has a limited set of values. The nearest to 500 is 470, yellow, violet, brown. It will dissipate about 200mW, so a quarter watt type will be OK, but several in a close space could start to get a bit hot.

That leads to the other advantage of using one resistor for a string of LEDs. Same current, less voltage to drop, less power to lose. With the same notional LED, but two of them, the LEDs account for 2+2=4 volts, leaving 12-4=8 for the resistor. 20mA at 8 volts gives 400 ohms, nearest is 390, orange, orange, white, brown. Wattage is down to 0.16 watts, the quarter watt rating will run much cooler, and there will be fewer of them. Stringing three carries on the same way, and so on. The limit is where the fluctuating voltage of the battery gives a big difference across the resistor, thus the current through it, thus the current through the LEDs, and a reduction in their brightness.