Deception effect calculation

[Liebchen]

The deception effect probability is mentioned in the User Manual under "deception effects," of course, but sadly nowhere else.

a. Is this effect probability based upon a single test, i.e. is the hex within deception range?

b. Or is it a "die roll" for each deception/partisan unit within range of the hex?

c. Or is it a single test, with cumulative effects from more than one deception unit?

In short, is there any benefit to having a hex within range of more than one deception unit?