Numbers: Count unique items in a column
Here is a clever solution posted in an apple discussions thread.
You basically create another column, and use the formula =IF(COUNTIF(A$1:A1, A2)=0,1,""). This will display a one for the first occurrence of each unique item. You then simply sum the whole column and you get your count.
If it doesn't have to be updated dynamically, you might also copy the cells and run something like pbpaste | sort | uniq | wc -l.