the field that i have relating them is client name
What will you do when you get two clients of the same name? What will you do when Mary Smith changes her name to Mary Jones? What will you do when you enter a client's name as John Smith and then discover at a later date that his name is really John Schmidt? If you use an auto-entered serial number field in clients instead of the name, these issues are avoided.
This is a rather weird method, but there's a solution named "sum the reciprocal" that will count the number of different clients listed in your found set of billing records: How to count the number of unique occurences in field.
If you have any questions about that method, please feel free to use the Post A Answer box below this message to ask them here instead of in that thread--where the context will be lost.
Thank YOU the sum the reciprocal worked.