How to add numbers to grouped rows in postgresql group by clause

With help from this question and its answers:

SELECT gid, capt, row_number() OVER (PARTITION BY capt ORDER BY gid) AS rnum
FROM your_table_here ORDER BY gid;

The row_number window function provides the count.

The PARTITION BY statement in the OVER clause tells the database to restart its numbering with each change to capt. The ORDER BY in the OVER clause tells the database to count along with the gid column.