Exact string matches considering Case with COUNTIF
It would be great if this could be accomplished with a single formula, but as of this writing Numbers does not support Array formulas.  

I was able to solve it by testing for "US" and returning "1" for true and "0" for false.


Formula in adjacent or other column: =IFERROR(FIND("US",A1),0) and drag formula down. (The FIND function allows for Case testing.)
Then using =COUNTIF(range,1) where range is the cells containing a "1" get the count.


Note: this can be made dynamic by changing the formula slightly.  If Find() is changed to look at a cell where the user can input the search criteria, then the result can be any of the variances.   =IFERROR(FIND([absoluteCellReverenceWithCritera],A1),0) - See last screenshots.