Option 1: 7 ducks

    Duck Duck < 2 in front
       Duck   < of a duck
       Duck   < duck in the middle
       Duck   < a duck
    Duck Duck < 2 behind

Option 2: 5 ducks.

    Duck Duck < 2 in front
       Duck   < duck in the middle
    Duck Duck < 2 behind

Option 3: 3 ducks.

    Duck A  < front of C
    Duck B  < front of C, middle, behind A
    Duck C  < behind A

If the interpretations of the riddle are mixed between clauses, any number between 3 and 7 is also possible.