What If: Dunbar's Number

Humans seem to have a limit on the number of social connnections that they can maintain in a community which is somewhere between 100 and 250; this is called Dunbar's Number (Dn). It's the size of a neighborhood, a medium-sized company or a tribe.

What if humans had a Dn of 20? Could societies build if a feeling of locality and common interests couldn't really extend to more people? What would cause/enable a value like that? At a first guess, humans would have to be much more self-sufficient.

What if humans had a Dn of 20,000?