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I am familiar with the physical explanation as to why microwaves heat up food: the rapidly fluctuating electric field exerts a torque on the dipole of the water molecules resulting in vibration and thus and increase in temperature.
As long as all the food in the microwave is exposed to the microwaves, why should it matter how many items are cooking at once? Is there only a finite region on which the microwaves are concentrated?
Lots of food labels contain different cook times based on the number of items you heat at once.