After Inserting , Update / Insert into another table in pl/sql trigger
Instead of materializing the count, which bears the risk of inconsistencies, I recommend you drop the my_tags table and create a view which uses aggregation to get the count.

DROP TABLE my_tags;

CREATE VIEW my_tags
AS
SELECT sr_no,
       tag,
       brand_id,
       count(*)
       FROM tag
       GROUP BY sr_no,
                tag,
                brand_id;


Like that you don't need any trigger, the count is always accurately calculated.