I've been listening to Episode 42 of DPP and around the 49:00 mark, Bart refers back to a point he's made before about opponents not being able to conceive of how many chips are in a cluster. I think the phenomenon Bart refers to is real, but I just remembered something that I think may be relevant to this concept.
When I worked for AT&T, I learned that the seven digit phone number system in the United States was no accident. Bell Labs (the R&D arm of AT&T), I believe, did research and found that seven digits was the highest amount of digits that people could reasonably be expected to remember.
Remember a discrete order of numbers is different than recognizing how many objects are in a pile, but it nevertheless shows the weakness of the human mind in dealing with more than a few units.
Here's an ABC News article that discusses "the magic number seven" and frames this in terms of "working memory" http://abcnews.go.com/Technology/brain-memory-magic-number/story?id=9189664