For me usually 2-3 would be the norm, although it could be infinite if I wanted to. In many poly relationships, there's a primary partner, and may be both of these primary partners "maintain" the third partner, so you are not alone in "maintaining" all the others. Or all the partners in the relationship can "maintain" each other in different combinations or all altogether. It might be easier to maintain the relationships in these cases. I'm not in one of these situations though. Currently it is 3 for me, with one primary, but I'm possibly nurturing more.