The main component in this circuit that will determine the output current is the C1=1µF capacitor ..
As you kno a capacitor represents certaun impedance for AC currents, and its value can calculated as follows:
Zc = 1/(ωC) = 1/ (2Πf*C), f=50Hz, C is the serial capacitor ..
For example, if you need 100mA of 220V the required resistance (impedance) is 2200Ω .. In this case, if you use 1.45µF capacitor, it will allow 100mA to flow through the circuit ..
Also, you need to take into account the output voltage, in this case regulated by 33V zener diode, and that the voltage is recified and smoothed by a smoothing capacitor ..
33/1.4 ≈ 24Vac is required to be recified and reulated at 33V level ..
So, 220V - 24Vac = 196Vac
To drop from 220Vac to 24Vac and allow 100mA current you will need Zc=1960Ω impedance and that can be done with 1.625µF capacitor (at 50Hz) ..
C = 1/ (2Π*50*Zc) ..
R1 of 100Ω has no special meaning and is used mainly as a "fuse" .. but you can add it to Zc if you wish ..
Regards,
IanP