How much current can I draw from one of the output of a transformer
Main limitation factor is temperature. Each coil is heated by its own current and they all heat the transformer as whole. Also, some heat dissipates in the core itself. So, you can trade the current of one output coil to another. But usually each coil is made by the wire of specific cross section, fitted to nominal current of that coil.
Total power rating of the transformer is a function of geometry size and working frequency, so it may be guessed. Then, that power divided by voltage of any output coil will give an estimation of the maximum current for that coil in case other coils run idle. Usually in such condition, that coil will suffer moderate overheating.