arduino uno external power supply
The Arduino itself has a limit of about 800mA. That is imposed by the voltage regulator that converts the incoming voltage to 5V.

If you need more than 800mA then you need to provide your own regulated 5V power source and feed it in to the 5V pin (not VIN).

Suitable power sources are mobile phone chargers (modern USB ones).  They generally come in multiples of amps.

And to answer your theory question - the current is the maximum that the supply can provide. It's the voltage that is fixed and the current varies depending on the load.