It's lonely behind the sofa.

Much like the law of conservation of ninjutsu, there's clearly a Law Of Inverse Dalek Scariness, though it's not strictly linear.

One Dalek is scary. Two Daleks are scarier. Three Daleks are scarier still, but diminishing returns start to set in. By the time you get to half a dozen Daleks, it's clear that the Doctor is going to come up with some clever or technobabbly solution that neutralizes all of them at once.