Betrayal Haunt #38 How Many Firebats?
Your interpretation was correct. There are several haunts where it refers to "as many things as there are players". While the sentence is grammatically ambiguous, the other interpretation would be very strange because there wil always be only 1 player in the room where the Haunt started; the player who revealed the Haunt. So if that's what they meant, then they could have just said "place 1 firebat in the room..."

Also, the newer revision seems to confirm that the number of firebat is meant to be based on the number of players; though obviously they felt the traitor needed to be nerfed.

One other "proof"... if "players in the room where the Haunt started" were meant as all one clause, then there's nothing at all in the sentence telling you where to place the firebats. The interpretation would be "place 1 firebat", with no mention of where to place them.