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

welcome to the forum. you can use:
=SUMPRODUCT((TEXT(Sheet2!$A$4:$A$24,"mmmyyyy")=TEXT(A2,"mmmyyyy"))*(Sheet2!$B$4:$M$24>0))/2

but i dont think that's a good way of storing your data. your columns will increase i suppose? xlsx files have only 16,000 columns but 1 million rows! you can check out the Modified sheet i attached on how to store your data. it will be cleaner & better for calculating. i did a formula version & also a pivot table easily