I'm not part of the culture either, but my money's on "totally depends on the people." As with vanilla relationships, you get people who are into varying degrees of polyamory.

In my head, c!Stephen would enjoy the dynamics of having multiple Doms (as long as they were well-coordinated enough not to give him contradicting orders, because that would freak him out), but get jealous easily if any of them had other subs. While l!Stephen would get twitchy if there were other Doms in the picture (although he might turn that itself into fantasy fuel, as in "some evil Dom kidnaps and hurts my sub, until I come to the rescue"), and in theory would enjoy the idea of having a whole passel of subs, but in practice has his hands full with the one.