I'm not sure on an upper limit, it really depends on how many people you are capable of loving. Personally I've never had more than 6 partners, although having 6 is daunting for me it's also exciting and fun. I love them all and they all make me happy, it's not difficult to find time for all of them, but I imagine that it is for some people. We have a lot of sleepovers, and meet up in town a lot, we all go to the same youth group. It just takes a lot of planning, thought and communications.
As long as there is communication I think any reasonable number is okay, reasonable meaning that you clearly can't love 291 partners or whatever abstract number. I don't want to say an upper limit, but I personally wouldn't consider going into double digits for relationships.
In a sexual respect, I usually don't even like three-ways very much, personally I prefer to sleep with partners separately, it's just more romantic and loving that way.
The difficulty with saying relationships like marriage is that some of my partners may have other partners, in which case we're going to need a pretty big house if we do get married, but that would be lovely if you ask me... just as long as there wasn't too many of us that it got confusing...