Distinct Combination of Two Columns
You can count distinct elements by running:

select count(distinct policy_id, client_id) from policy_client;


Another option would be to group by and count that:

select count(*) from (select policy_id, client_id from policy_client group by 1,2) a;


Run both version and see which one performs better on your dataset.

A very quick way but not totally accurate if you have a key on (policy_id and client_id) you can also check the cardinality of that index but that's an approximate not exact number.