Does "number of heroes when the haunt was revealed" include the traitor? (Haunt 17)
When the haunt was revealed, one player becomes a traitor (or in some scenarios, is removed entirely), and the rest become heroes.


  Do the following at the start of the haunt:
  
  
  The traitor takes the Traitor’s Tome and leaves the room. The traitor reads only the haunt that is starting now.
  The rest of the players become heroes. They look up the haunt with the same number in Secrets of Survival and read it together. (The heroes should also
  talk briefly about their plan for survival.)
  
  
  [...]


So if the game started with three players, two of them become heroes. There weren't any heroes before that.

"The number of heroes when the haunt was revealed" is used instead of "the number of heroes" to avoid having the goal become easier as heroes are eliminated.