Dynamite, I agree with you that the 42 circuit limit seems arbitrary- and should be deleted.
I believe the number of circuits in a panel is a design issue. Personally, I prefer a main panel serving a galaxy of satellite sub-panels.
I prefer multiple subs for two reasons: the breaker should be located close to the area it serves, and there is a limited number of openings you can put in a panel case.
Using your example, 84 romex circuits would need at least 42 1/2" knockouts- under the best of circumstances, 21 top and 21 bottom. That many holes needs a lot of metal!
