Hey there, I've personally run that adventure for a group of 4 with 1 or 2 NPCs appearing.
However, I purposely don't try and limit adventures to specific size groups or levels and would encourage the referee to make adjustments to adventure (i.e. simple examples like numbers of monsters) to ensure this is a challenge for the players and there is a sense of danger. In some cases, I like the idea of a monster being too tough to fight head on and players being encourage to plan around this.
