How Many Characters Do You Kill in a Thriller Novel?

I've had this question posed to me recently and I actually had to stop and really think about it. 

How many characters are too many to kill off in a thriller novel? 

Is there a minimum number of murders to a thriller novel?

My first answer was… I'm not really sure.  I don't think there is a special number of appropriate deaths for thriller or suspense novels.

For thriller novels, it's just part of the creative process to kill off characters, likeable or detestable, and even just a planted...