Too many? No such thing. For ME I don't want to add anyone to the relationship, but that has nothing to do with a rule about it being "too many".
I would say that depends on the individuals involved and the dynamic of the relationship. There's no hard and fast rule that would apply to every poly relationship. I know people with VERY big relationship networks. Random made-up example, A and B are a couple, B and C and E are an equal triad, and E is with B and F, A is also with F, etc. etc. That WORKS FOR THEM and that's their business. I couldn't set a "rule" for how many is too many for them. That's as bad, to me, as saying two men shouldn't be together or whatever.
I'm in an equal triad, and we're happy like this -- for now. That doesn't mean it will always stay that way, of course!