How to calculate the output amperage of transformer
You start by specifying what power you want through the transformer. A 1kW transformer needs much more iron than a 6 watt.

Given the power, you then choose a suitable core. Broadly, power goes as weight of core, over quite a wide range. Core manufacturers have tables that shows you core size required for any given power rating. 

Only after you have the core size, then you can work out the primary turns. Too few and the transformer won't work, it will blow fuses or burn. Too many, and it will be inefficient. Calculate the primary turns based on your input voltage, and swinging the core over a 'reasonable' range of flux, typically +/- 1.5T for iron. Size the primary wire to fill half of the winding window.

Now you can calculate your secondary turns, as (Vout/Vin) * primary_turns. Size the secondary wire to fill the other half of the winding window.

If you have started with the right size iron core, then filling the winding window with copper (allowing for insulation, the bobbin, and a few gaps) will 'just work' to give you enough current handling capacity to meet your power specification.