Here is my take on it. I think you are looking for a hard limit that is person to person. Where you ACTUALLY run into problems with large constellations is the people factor. Your have exponentially more socializing requred with larger groups. Unless someone is dating and keeping it private (which does happen)
As a triad you have 4 relationships
Quad has 7 relationships (maybe more my math is off today)
Thats a lot of freaking relationships and time.. and ALL of it completely within this circle of people. Trying to manage all of those relationships not only become tedious, but almost impossible. Not everyone is going to get along with me... add a 5th person, you end up more complex. Then of course you have friends and family. The time invested becomes... ridiculous. I am a private person, when the hell do I get time for me?... for example
So even if I can, in theory, love endlessly, I sure as hell am not going to love everyone enough to put up with all the metamours involved.
I think thats why and when you run into sizing problems in regards to poly.