Software "Interconnect" complexity

In a given, nominal task (collection of related threads), how many
"external interfaces" (hooks to/from other tasks) do you typically
have?

Complexity is best managed when the number of such interactions is kept
small and focused.  But, what is a good target metric for this?

[Clearly, each task needs to interact with at least one other task/driver
to do meaningful work.  But, at what point is it's focus too "scatterbrained"
suggesting a refactoring is indicated?]