I am quite uncertain as to how many partners I can handle. I've always felt there's been "room" for more in my life, though I've never been in a situation where everybody was local and interacting on a daily basis. I can guess three or four because I can easily imagine that as workable; that's still very much a guess, however.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.