Excellent initiative, Chris, and timely, since I just refurbished the 100 Circles page, hoping it's now more readable than it was.
My almost reflex answer to "how many degrees?" is "seven". Because of my aunt Sidonie's saying : On hérite de sept générations. A sentence I've long wondered upon (see this page if you read a bit of French), and the more I think about it, the more I consider there was a deep truth in this number.
If you are born, say in 1960 (average WikiTreer I would say), your 7th generation ancestors were born by 1750. Finding all your ancestors at that generation (128 default pedigree collapse) is a reasonable target.
Our reference profile HM the Queen has currently about 8,200 profiles in her 7 first circles. And over 67,000 in the 10th. Way too much for the target objective.
Yourself Chris have, as I write, 2,765 up to C7, and 24,303 in C10. Other examples would consolidate this choice of 7 as the magic number.
A contentious point is that challenging WikiTreers to augment this number might conflate with privacy concerns. A divorced WikiTreer would be, or not, depending on circonstances, eager to develop her former spouse's first circles, although, or because, they could augment radically the score. And WikiTreers with private profiles would feel uneasy with such an idea.
A workaround to this would be to be able to choose to compute the score, not for the circles of the competing WikiTreer, but for one of her recent ancestors, in the first generations with open profiles. In the spirit of the "Connect 1900" initiative, I would suggest to pick an ancestor living by 1900. I would gladly pick my grandmother Catherine Favennec.
[edited] : another thought : Having this number on all profiles, not only WikiTreers, would be a good idea. Figuring at first sight if a profile is well connected or not would be helpful. Maybe this would be resource consuming, though.
[edited] : see also my comment at the top of the thread for an alternative proposal, displaying the 10k Circle.