In a six player game, you only need 3 successes (half the number of players). These are put on the board when a successful Knowledge or Sanity roll is made. This allows the heroes to pick their stronger stats. The Heroes need 3 successes, the traitor needs 2 successes, but he needs one of each.
The rest proceeds from there...
I realise I'm bumping an ancient thread (or... resurrecting... MWAHAHAHA), but having just played the Séance want to make sure people aren't misled here.
In fact, in a 6 player game you need SIX successes. You draw 6 knowledge roll tokens + 6 sanity roll tokens, and then the remaining heroes need to get any combination of 6 successful roles. (We got lucky and had a very Knowledgeable Doctor in the house who made a lot of knowledge rolls.)