There is no one answer. It varies from one power supply to the next.
Single rail,Dual rail,multiple rail.
But all have a set limit on the 12v rail(s).
On a single rail it is governed by the wire guage used and over current protction.
On Dual rails it could be 18amp per rail maximum, but a total of 30amp maximum. Meaning if one is pulling 18amp the other can only have 12 amps.
Multiple rails could be one single rail with over current protection on each rail. Or it could be 3 or 4 individual rails.