Andy,

I agree with you that the term "limiting values" is confusing, it should be Maximum values. Also in that same table it says under V0h that you can get 20 mA which I believe to be false. This would imply that you could source 20mA per pin, with no voltage drop, this is false.(See Kai's response on calculated transistor ON resistance)
The general rule of thumb for I/O pins is :

Sourcing = 5mA per pin

Sinking = 20mA per pin
This is based on the transistor circuitry that is used for the I/O pins, which is pretty much standardized.
The number of pins that can be used simultaneously is dependent on the Maximum Current allowed for that chip.

Regards,

Jacob