Problem adding a number of students registered to a course (as a row) to a SELECT statement in POSTGRES

I have a database of courses. I need to get a name, a a topic, a teacher, a duration and a number of students registered. I get the first four successfully, but not the last one.

Here is what my tables look like:

courses table

journal table

all tables

That's the successful part for the first four:

SELECT c.name, t.topic_name AS topic, u.name || ' '|| u.surname AS TEACHER, ((c.end_date - c.start_date) / 7)::int AS duration
FROM public.course c 
RIGHT JOIN public.topic t ON  c.topic_id = t.topic_id 
RIGHT JOIN public.teacher_course tc ON  c.course_id = tc.course_id 
RIGHT JOIN public.user u ON tc.teacher_id = u.user_id 
WHERE u.role_id = 2;

Basically, to know the number of registered students per course, I only need to count records in the journal table for each course, but when I add count(j.id_record) AS students_registered

it just breaks and asks me to group everything by and blah blah. I'm confused about that. How to get this number correctly for each course?

How many English words
do you know?
Test your English vocabulary size, and measure
how many words do you know
Online Test
Powered by Examplum