Attempt by Lament (I don't currently have a Senseis account):
Eight moves, of course:
Black always passes.
If the problem implied that the solution should contain no passes, then there're several different 12-move solutions, and it seems that no solution can be shorter than 12 moves (since black has to spend 6 moves to build a B2 Bomber that White captures with the final move.
More than 12 - Remember W can't pass while she's building up the bomber! Reuven
Is suicide allowed?
ferdi: No suicide, no passes, please :-) I added it to the problem page, too.