Just how many black sheaths does a girl need?

I'm drooling over this dress from White House Black Market and am constantly tempted to order it.

But then I remember that I own:
- A black mid-knee-length sheath with elbow-length sleeves (and a lovely wide square neck)
- A black knee-length sheath with an asymmetric neckline and a front slit
- A simple sleeveless black knee-length sheath

And of course:
- A mostly-black colorblock sheath with 3/4 sleeves
- A solid purple sheath with cap sleeves
- A navy puff-sleeved sheath
- A royal blue sleeveless sheath with a slim v-neck


But I don't have any dresses that look quite like this!  Oh if only this were dark grey.

It's time for a closet cleanout, and if I end up getting rid of a few dresses, I might let myself add this one.  But I have to do a thorough cleanout first and see just what's left in there!  Even I think that 3 black sheaths is probably enough.  ...  Probably.