This is discussed in The Universal History of Numbers, by Georges Ifrah. You better check the book, because it has been a long time since I read it, but he exp

This is discussed in The Universal History of Numbers, by Georges Ifrah. You better check the book, because it has been a long time since I read it, but he explained that most people counts 3 by 3, sometimes 2 by 2, and exceptionally 4 by 4. Only very exceptionally people can count 5 by 5. What most of us do when counting 5 elements, is to divide them in a chunk of 3 and one of 2.