Determining the Maximum Output Current of a Switch Mode Power Supply
In general, for a given power supply:

For DC-DC:
First, look at the maximum power available to the input.  If your input can supply X watts, then the most output power you can supply is X*efficiency.  Switching supply efficiency usually runs in the range of 70% to the 90's depending on a lot of factors.  You'll also need some margin for transients, line tolerance, and possible unknown factors.

Second, you need to design your converter for a specific current.  If you pick a DC-DC converter IC, it will have parameters on the datasheet for max current, current limit, thermal performance.  All these things have to be taken into account in designing for a particular output current.

If you pick a controller IC with an external switch then you also have to select the switch (likely a FET) correctly.

For AC/DC:
For an AC/DC converter things get much more complicated.  You have high voltage and isolation to worry about, creepage and clearance, a much harder job in the layout, EMI concerns, wide input range, holdup time, and component selection.  None of these things are trivial.

A SMPS is not a simple thing to get right, there are lots of pitfalls, especially in layout.  An AC/DC converter is not a good beginner project when you can buy an off-the-shelf unit for less than you could build one, and save yourself and anyone using your device from possible injury.  

If you have to ask how to determine the maximum current output you're not ready to build an offline converter.  Start with a low voltage DC-DC converter, then when you have learned all you can about power electronics decide if you want to start learning enough to build an AC/DC converter.