Apple - Numbers: Count unique items in a column

If it doesn't have to be updated dynamically, you might also copy the cells and run something like pbpaste | sort | uniq | wc -l.

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.