"These twice five figures" with which we count are clearly related to our five fingers[0].
But why five? TIL that it's a few hundred-million-year backward compatibility issue:
Because there are only five distinct Hox-encoded domains across the limb bud there is a developmental constraint prohibiting the evolution of more than five different types of digits.
[0] at least when counting moving objects, such as cattle; bakers, whose objects tend to stay where one left them, use nicer rectangles, and hence prefer dozens to tens.