simple question...in the Galactus and Spectre Scenarios how many objects do you choose , and who decides where they go ?

Same as any other game. The only difference is there's a Big Fig sitting in the middle of the map.

Something to keep in mind for any scenario (not just the Spectre or Galactus ones, but the WK special events, too) is that unless the scenario rules specifically mention something about the set-up part of the game being different than normal, it isn't different. You still follow all the standard rules on pages 9-12 of the Icons rule book.