how to increase current in this given circuit
First, and most important, this circuit is not safe. A mistake putting it together can kill you or someone else. Do not use it for anything.

It definitely will not work well at 2 amps. Zener diode regulators work by converting the extra current capacity of the power supply to heat. 2 amps at 12 volts is a huge amount of power to waste.

Passerby's answer is the right one. Buy an AC-DC converter. Don't try to build your own if you don't have the knowledge.