Count number of checked boxes

I am having issues coming up with a formula in the "red box"that will display a number 1-8 depending on how many boxes are checked with an "x" in cells C2-C8 and then reversing that number.

Example, if 8 boxes are checked, the red box should display 0.
Example, if 0 boxes are checked, the red box should display 8.
Example, if 3 boxes are checked, the red box should display 5.

Thanks in advance
Robert