The problem here probably isn't so much that there's a limit, but that corporate HQs that compete with one another can't be co-located in the same city. So if you have Mining, Inc. headquartered in a city, you can't also establish the HQs of Creative Constructions and Civilized Jewelers in that city, too, because they compete with each other for the same resources. I don't think there's an easy way around that.