Hi there, I've been working on the same question. This is best solution I've come across so far, found here at office.microsoft.com

I feel like there must be a simpler solution, so I'll be watching to see if one pops up

applied to your sample file:
Copy of Sample File-1.xlsx

This is how I've used it:

=SUM(IF(FREQUENCY(IF(LEN(B4:B4000)>0,MATCH(B4:B4000,B4:B4000,0),""), IF(LEN(B4:B4000)>0,MATCH(B4:B4000,B4:B4000,0),""))>0,1))
It is an array formula, so be sure to hit cntrl+shift+enter

jenny