Postgres 11

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table subscriber (create_timestamp timestamp, subscriber_id int);

insert into subscriber values ('2021-01-01', 3), ('2021-02-01', 12), ('2021-04-01', 4)
, ('2021-04-11', 5);
4 rows affected

SELECT t.cdate
     , COALESCE(p.total_count, 0) total_count
FROM (select generate_series('2021-01-01'::timestamp, '2021-12-15', '1 month') as cdate) t
LEFT JOIN (SELECT DATE_TRUNC('month',create_timestamp) create_timestamp
                , SUM(subscriber_id) total_count
           FROM subscriber
           GROUP BY DATE_TRUNC('month',create_timestamp)) p
       ON t.cdate = p.create_timestamp    


cdate
total_count


2021-01-01 00:00:00
3

2021-02-01 00:00:00
12

2021-03-01 00:00:00
0

2021-04-01 00:00:00
9

2021-05-01 00:00:00
0

2021-06-01 00:00:00
0

2021-07-01 00:00:00
0

2021-08-01 00:00:00
0

2021-09-01 00:00:00
0

2021-10-01 00:00:00
0

2021-11-01 00:00:00
0

2021-12-01 00:00:00
0


SELECT DATE_TRUNC('month',create_timestamp)
FROM subscriber


date_trunc


2021-01-01 00:00:00

2021-02-01 00:00:00

2021-04-01 00:00:00

2021-04-01 00:00:00

cdate	total_count
2021-01-01 00:00:00	3
2021-02-01 00:00:00	12
2021-03-01 00:00:00	0
2021-04-01 00:00:00	9
2021-05-01 00:00:00	0
2021-06-01 00:00:00	0
2021-07-01 00:00:00	0
2021-08-01 00:00:00	0
2021-09-01 00:00:00	0
2021-10-01 00:00:00	0
2021-11-01 00:00:00	0
2021-12-01 00:00:00	0