Ok, I will give a short answer tonight but plan on posting a full explanation of this in the near future.
The cortex has two paths for current to flow from the battery to the motors. Motor ports 1-5 are on one circuit, motor ports 6-10 are on the second circuit. Each circuit is protected by a 4A PTC (positive temperature coefficient), a type of resettable fuse. I posted a lot of information about the cortex PTC in this thread, although I have not had time to finish the tests.
The motors will use a variable amount of current depending on how much torque they have to provide. The specs for the motors usually give the no load current (maximum rpm) and the stall current (motor is stopped), you can create a graph that plots the current against rotational speed of the motor that is usually close to a straight line meaning that at half speed the motor will draw about half the stall current. See here for some information on this. For the 393 motor the stall current is given as 3.6A so when it is running at half speed the current draw will be about 1.8A.
If you had 4 393 motors running together at half speed then the total current needed will be 4x1.8 = 7.2A, as you can see this is above the 4A that one circuit on the cortex can handle so the PTC will protect that circuit after a few seconds if all the motors were all on one of the two groups. Even if you spread the motors over the two circuits, you can blow the PTC if the motors were to stall and remain so for several seconds.
All of this can get even more complex when you also take into account the fact that the motors each have their own PTC but the following facts will not change.
The cortex can supply 8A for about 30-60 seconds spread over it’s two circuits.
If you use a power expander you can have an additional 4A from a second battery.
You can run the cortex beyond the 4A per circuit but it will be for a much reduced time, for example, tests show that you can pull double the current for about 6 seconds.