I am working with NodeMcu ESP8266 but I want to implement it in a power supply such as a cell phone charger, I still don't know how many mAh it can work without breaking down.
Update the charger is 5v 2.1 mAh
Make certain that the voltage is correct, and that the power supply can provide more than the required current (mA), and you will be fine.
mAh is a measure of battery capacity.
If the voltage is correct electric and electronic devices only take the amount of current that they need. Having a high capacity battery is no problem.
...R
Note that current and capacity are separate things, not to be confused.
Update the charger is 5v 2.1 mAh
update: no its not. It might be 2.1 amp, but chargers don't have a capacity as they are powered from the mains which doesn't go flat!