Try:
SELECT c.id as ID,
c.name as NAME,
cu.notification_level as NOTIFICATION_LEVEL,
CASE cu.notification_level
WHEN 0 THEN 'MUTED'
WHEN 1 THEN 'REGULAR'
WHEN 2 THEN 'TRACKING'
WHEN 3 THEN 'WATCHING'
WHEN 4 THEN 'WATHING FIRST POST' END as NOTIFICATION_LEVEL_DESC,
COUNT(cu.id) as COUNT_OF_TYPE
FROM category_users cu
JOIN categories c ON cu.category_id = c.id
GROUP BY c.id, c.name, cu.notification_level, notification_level_desc
ORDER BY c.name, cu.notification_level