Hello,
I am trying to solve transformer's circuit problem. In input I have 12V AC voltage, I made calculations, by using Kirchhoff's law, and found that voltage on last capacitor are 4V. I want to connect these 4V to transformer and get 20V in output. Transformers output load should take 1A of current, so in input shoud be 5A. So my question is how I suppose to calculate current from supply (I in scheme)?

 

What are you trying to do?
Why all the resistors and capacitors?
Do you have a reason to not just use a transformer?

 

I too am wondering why use the resistor-capacitor circuit to feed a transformer. It is not a realistic way to reduce a voltage in a power system. In addition, connecting a transformer would change all of the voltages and currents in the system quite a lot.
Do you already have a transformer? To get 20 volts out from 4 volts in requires a 1: 5 turns ratio, which is a bit uncommon for power transformers.
OR is this a homework assignment?