You don't need to change C1 or R2 to use 20 LEDs.
The current is controlled by C1, and the voltage and frequency of the supply. Going from 1 to 20 LEDs increases the voltage from 3.6 to 72 V but compared to 220 V either are quite small.
220 nF will give about 15 mA with 220 V and 50 Hz.
R2 is just to discharge C1 once the circuit is unplugged. The value is fairly arbitrary. 1 MΩ will discharge the C1 in less than a second. You could probably use a larger value, but values beyond 1 MΩ may be harder to find.