Buck converter output current
it only makes 1A


No, It can output 3A with carefully heatsinking.


  I have no clue how to calculate how much amperage the buck converter will make


"will make " or "will consume "?

Here is the graph showing efficiency vs input voltage :



As you can see, the efficiency is about 80 % if input voltage is 12V. 
The only problem is if your 12 V adapter can provide enough wattage for your load.