I have read a lot in here about counting unique cells. However, I need to completely disregard duplicate values when counting.

For example, column A is a list of customer numbers. Each row of data represents an order they have placed. I need to count the number of customers who have only made one order. With the array formulas I am reading about for counting unique values, even the customers who have ordered more than once will be counted once.

Column A
655
767
878
655
456

Using the "count unique values" formula I have seen on many other threads, the count for the above data would be 4. I need to completely disregard the "655" and end up with a total count of 3 completely unique values.

Any help on this would be greatly appreciated.
Please let me know if I can clarify anything, or if you need any more information.