This is a hard question. I think this evolves with time. Right now I have two partners, which feels plenty and I have no desire for more. But I do not think I could have started a second relationship some years ago, because I doubt either me or my husband, or our relationship, would have been ready for it at that point. Thus, there was fullness with one relationship. I can't say precisely when it is that the situation changed, but when I met my girlfriend we were lucky enough that both of our marriages were in a place that made poly possible.
At the moment, making this situation work is taking all my energy and concentration, and I definitely am full. Also, it would be way too soon for my girlfriend, and our relationship, for me (or her) to have a third partner. And, me having a girlfriend is a big adjustment for my husband, which is why it would be too soon for him as well, and to the relationship between us.
So, right now there are many reasons why I'm full, both within me and in my relationships. But the relationships will evolve and settle and so the situation might allow for a third partner at some point. I had been with my husband for about 7 years before our relationship became poly. So, maybe another 7 years until the relationships are ready for it; and then we'll still have to see if I have time/energy etc.