Hello,
Assignee field in your "client" table must be a linked field to an "Assignee" table (one row for each assignee in this table):
Once you have done that, you can add a "count" type field in the "Assignee" table to get the number of clients for each assignee:
Field Nb Clients is defined as such (company being the client table in this example):
Note that Assignee table can be created directly from you client table by changing Assigne field type to "link to another record"; you will then have the possibility to choose the option "create a new table".
Regards,
Pascal