It's the Unique counts--so that you do not count the same client twice in the same fiscal year that's making this tricky.
To count unique values (client ID's for a given fiscal year), you can use:
Execute SQL: A new way to count unique values in FileMaker 12
Sum The Reciprocal: How to count the number of unique occurences in field.
Or you can set up a self join relationship that matches tickets to a new occurrence of tickets by fiscal year (field must be stored/inxed) along with a conditional value list based on that relationship so that you can use ValueCount ( ValueListItems... to count the number of clients for that fiscal year. (you can also use a new occurrence of Clients to get the same count.)