I usually refer to it as the "kid in the candy store" phase of discovering polyamory
Oh the possibilities out there, once you begin this journey of open loving and it's so difficult to pick and choose once you don't have
Each of us probably has our own individual limits on how much time and energy we have to devote to multiple relationships and no one can really tell you what yours is.
Through trial and error, I've come to believe that my own limits are probably two "primary-type" relationships at the same time, with the possibility of maintaining a couple of less daily-life-entwined love connections.
I do want to echo LovingRadiance's take on starting multiple relationships at the same time though. My personal experience has been that each relationship is better served by having its own time to develop. I feel the need to know that each relationship has a solid foundation before adding others to my life and that takes time to establish.