What is the best way to count the number of clients from a table?

Hi all,

I'm after some help counting how many clients have made a purchase over the months. Generally I could just do a count of the number of rows that contain text per month & year but each client has a couple of columns so I would be double counting.

Below is the start of my formula, I am able to find the correct month & date, but I need to know what is the best way to count the number of clients that have made a purchase with me for each month.

IF(AND(MONTH($A2)=MONTH(Sheet2!A11),YEAR($A2)=YEAR(Sheet2!A11))

I've attached a sample spreadsheet that shows what I am after, please note my actual spreadsheet is huge with dates going back to 1995.

Any help is much appreciated!!