Oracle 18c

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

alter session set nls_date_format = 'yyyy-mm-dd';
create table tbl (Name, Date_, Count_) as (
    select 'P', date '2020-01-01', 4 from dual union all
    select 'P', date '2020-03-01', 3 from dual union all
    select 'P', date '2020-05-01', 2 from dual union all
    select 'P', date '2020-06-01', 5 from dual union all
    select 'T', date '2019-12-01', 4 from dual union all
    select 'T', date '2020-01-01', 1 from dual);
6 rows affected

with dates as (select min(date_) mnd, max(date_) mxd from tbl),
  cal as (select add_months(mnd, level - 1) mth
           from dates connect by level - 1 <= months_between(mxd, mnd))
select * from cal left join tbl partition by (name) on (date_ = mth)


NAME
MTH
DATE_
COUNT_


P
2019-12-01
null
null

P
2020-01-01
2020-01-01
4

P
2020-02-01
null
null

P
2020-03-01
2020-03-01
3

P
2020-04-01
null
null

P
2020-05-01
2020-05-01
2

P
2020-06-01
2020-06-01
5

T
2019-12-01
2019-12-01
4

T
2020-01-01
2020-01-01
1

T
2020-02-01
null
null

T
2020-03-01
null
null

T
2020-04-01
null
null

T
2020-05-01
null
null

T
2020-06-01
null
null

NAME	MTH	DATE_	COUNT_
P	2019-12-01	null	null
P	2020-01-01	2020-01-01	4
P	2020-02-01	null	null
P	2020-03-01	2020-03-01	3
P	2020-04-01	null	null
P	2020-05-01	2020-05-01	2
P	2020-06-01	2020-06-01	5
T	2019-12-01	2019-12-01	4
T	2020-01-01	2020-01-01	1
T	2020-02-01	null	null
T	2020-03-01	null	null
T	2020-04-01	null	null
T	2020-05-01	null	null
T	2020-06-01	null	null