The title of this entry comes from a Google search a recent visitor to my blog used.
I can't answer it.
The only people who can give you some kind of answer to this are senior faculty in your department (and possibly college), and yourself.
But you absolutely must know the answer very early in the tenure-track process. Preferably before you officially start your position. At the very latest six months in.
So how do you find out how many papers you need? You need to do two things:
1) Ask. Ask early and often. Ask the senior faculty in your department. If you've met some, ask senior faculty in related departments within your college (e.g. within medicine, or within arts and sciences). You may not get a straight answer. You may get several different answers. Hopefully you will get some kind of answer.
2) Look it up. Find out how many publications the last few people who made tenure in your department had. No one tenured in the last few years? Look up the publication totals of recently tenured people in related departments within your college.
Once you've done both of the above, take the largest of the answers you've found (they may not match up).
That's the absolute minimum number of publications you are aiming for.
Absolute minimum. You want more. You may need more.
The answer to the question "how many papers needed for tenure?" is not set in stone.
[UPDATE] Do apply some common sense to the above. Let's say your senior colleagues provide you with a consensus estimate of X publications, and the last three people to get tenure in your department had (X+1), (X+2), and (X+10) publications. Take (X+2) as your minimum goal.