The answer is that there is no direct relationship in your case.
When you stitch, you virtually increase the size of your sensor, simulating what you can get with a medium format or even with a - yet to exist - large format sensor.
I have 2 meters large stitches made from a 24mp d3x + Zeiss 100mm f2.0 that are in the 300~400 mp and leave very far behind any print I have seen from single captures done with top notch medium format backs.
Needless to say, much better can be done nowadays with the latest sensors from 5Ds/a7rII/D810 with the best glass such as the Otus lenses, but your 1dsIII is still a very good camera and stitching can bring it ahead in image quality compared to any of the latest cameras in the hands of a lazy person not interested in stitching.
Cheers,
Bernard