My understanding is that supply restrictions occur at two times:
When the supply card is drawn in the Westeros phase, and supply tracks are adjusted and armies reconciled.
During mustering, you can't create an army that would exceed supply limits.
However, the question is - is it within in the rules to say, during mustering create an army that does fit supply limits, and then use a march order to merge it with another army?
Or do supply limits also apply when applying march orders?