Welcome to our forum.
Perhaps it's the amount of monogamous conditioning? in our society ... Who knows, but as it happens, I'd say three-adult arrangements are the most "common" type of poly configuration you can find ... four-adults happens, but not as often ... five-adults or more seems to be downright unusual. And people don't always share the same household.
Having said that, I do know that "Sex at Dawn" is a highly recommended book, and may say something about where we (as a society) are headed in the future.
For now, I would suggest just soak up a lot of Polyamory.com ... Look through the various threads and boards; see what calls to you ... Post whenever you have questions. Keep in mind that everyone is different, and poly people can be quite unique.
So what works in one polycule, may not work in another. You have to find out what works for you.
Communication, honesty, and compassion, are probably the three biggest staples that "successful" polyamorists rely on. So seek to build on those three things.
Glad you could join us, and hope you will enjoy your stay.