MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
select version();


version()


8.0.36


CREATE TABLE users (id int, user_id int, login_time datetime);
# the data in you original question (https://stackoverflow.com/questions/70289093/find-mysql-concurrent-user-per-hour-in-a-week)


INSERT INTO users VALUES
(1,    23,     '2016-06-08 09:10:00'),
(2,    24,     '2016-06-08 08:55:00'),
(3,    25,     '2016-06-08 09:29:00'),
(4,    26,     '2016-06-08 09:40:00'),
(5,    27,     '2016-06-08 09:08:00'),
(6,    28,     '2016-06-09 13:40:00'),
(7,    31,     '2016-06-09 14:04:00');

Records: 7  Duplicates: 0  Warnings: 0


WITH RECURSIVE cte1 AS (
   SELECT 
      DATE(MIN(login_time)) StartDate,
      DATE(MAX(login_time)) EndDate
FROm users),
cte2 AS (
   SELECT cte1.StartDate
   from cte1
   union all
   select DATE_ADD(cte2.StartDate, INTERVAL 1 DAY)
   from cte2
   cross join cte1 where cte2.StartDate < cte1.EndDate
),
cte3 AS (
   SELECT StartDate, 0 as H
   FROM cte2
   UNION ALL
   SELECT StartDate, H+1 FROM cte3 WHERE H<24
)
select * from (
select 
   StartDate as `Date`,
   H as `hour`,
   (SELECT count(*) from users 
	WHERE login_time BETWEEN DATE_ADD(StartDate, interval H HOUR) AND DATE_ADD(StartDate, interval (H+1) HOUR)
   ) as `Count`
from cte3) x
where x.`Count` <>0
order by 1,2;


Date
hour
Count


2016-06-08
8
1

2016-06-08
9
4

2016-06-09
13
1

2016-06-09
14
1


# the data that you gave in a link to sqlfiddle (http://sqlfiddle.com/#!9/67356f/2)
delete from users;
insert into users  VALUES (1, 31, '2018-04-01 13:00:00');
insert into users  VALUES (2, 32, '2018-04-01 8:00:00');
insert into users  VALUES (3, 33, '2018-04-01 8:30:00');
insert into users  VALUES (4, 34, '2018-04-01 8:40:00');
insert into users  VALUES (5, 38, '2018-04-02 3:00:00');
insert into users  VALUES (6, 39, '2018-04-02 3:40:00');
WITH RECURSIVE cte1 AS (
   SELECT 
      DATE(MIN(login_time)) StartDate,
      DATE(MAX(login_time)) EndDate
FROm users),
cte2 AS (
   SELECT cte1.StartDate
   from cte1
   union all
   select DATE_ADD(cte2.StartDate, INTERVAL 1 DAY)
   from cte2
   cross join cte1 where cte2.StartDate < cte1.EndDate
),
cte3 AS (
   SELECT StartDate, 0 as H
   FROM cte2
   UNION ALL
   SELECT StartDate, H+1 FROM cte3 WHERE H<24
)
select * from (
select 
   StartDate as `Date`,
   H as `hour`,
   (SELECT count(*) from users 
	WHERE login_time BETWEEN DATE_ADD(StartDate, interval H HOUR) AND DATE_ADD(StartDate, interval (H+1) HOUR)
   ) as `Count`
from cte3) x
where x.`Count` <>0
order by 1,2;


Date
hour
Count


2018-04-01
7
1

2018-04-01
8
3

2018-04-01
12
1

2018-04-01
13
1

2018-04-02
2
1

2018-04-02
3
2
Date	hour	Count
2018-04-01	7	1
2018-04-01	8	3
2018-04-01	12	1
2018-04-01	13	1
2018-04-02	2	1
2018-04-02	3	2