Based on my experiences when I was active in the local bdsm communities I would say that the most common true, poly situations were for femdom's to have several male subs, sometimes a mix of male and females. Whereas poly male doms tended to be more into keeping a pair of subs. I have no idea idea why that would be, but it was what I observed to be the way of the real poly thing.
What I mean here is these poly D/s groups were long term, they attended events as a group of 3 or 4 (or more) all the time and generally played together regularly. Very few, that I was aware of, actually all lived together but they did all spend a lot of time together and for that reason were considered poly.
What is more common is for D/s couples to have additional casual play partners, be it for one off sessions or semi regular. I don't really see that as poly though as even in the case of semi regular it tends to be like a session meet up once a month or fortnight maybe.
This is how dungeon meets often work, it's kind of like a swingers club for kinky folk. You go along and subs may often get loaned out for the evening or swapped or grouped etc. There is not really a relationship other than these are usually people you already know and trust hence are willing to play with.
Another situation where this can play out is kinky gangbang style events. Here a dom may take his sub along to be used by many people. The one sub can be the center piece of the evening and be servicing a dozen or more other people. Obviously this again isn't really poly, it's simply using the situation for bdsm activities with no ongoing relationship involved.
From your description of your fantasy it sounds much more like what you want sits in the second category I described.
