Is a 2 Ampere power supply dangerous?
Since you have clarified your question, the answer is as follows:

2A is the maximum current it can deliver.  

It will always deliver 12V - as long as you don't put a load on it such that it would have to deliver more than 2A.  

Then, the voltage will drop. Either because it over heats and burns out or because the engineer who designed made it so that it safely limits the current so as not to burn out and be a fire hazard.

The current that really flows depends on the resistance of the connected load and the voltage.

Ohms law says this E=IR, where E is voltage, I is current, and R is resistance.

It can be rewritten like this:  I= E/R.

This is what we want, since it is the current that can kill you.

Your body can be thought of as a resistor.  It has a resistance of several thousand ohms.  It can vary from 1000 ohms to 100000 ohms.  

Lets stick with the middle field: 10000 ohms.

I= E/R

I= 12/10000

I= 0.0012A

I=1.2mA

So, your power supply can deliver 2A, but it can only force 1.2 thousandths of an ampere through your body.

So, your powersupply is safe for you to use and to touch with your hands.



Always buy your power supplies from trustworthy sources.  Switching power supplies must be properly made in order to isolate the ouput from the high voltage input.

A cheaply made powersupply might skimp on the isolation, and allow the output to be at a high voltage compared to the ground.  There will be the rated voltage between the two output terminals, but the full line voltage between one of the outputs and ground.  THAT can kill you, but has nothing to do with the rated output current or voltage.  That kind of thing is poor design and manufacturing.