users:               categories:
╔═══════╦═══════════╗╔═══════╦═══════════╗
║  id   ║ user_type ║║  id   ║ name      ║
╠═══════╬═══════════╣╠═══════╬═══════════╣
║ id 1  ║ tutor     ║║ id 1  ║ name1     ║
║ id 2  ║ other     ║║ id 2  ║ name2     ║
╚═══════╩═══════════╝╚═══════╩═══════════╝
ads:
╔════════════╦══════════════╗
║ creator_id ║ category_id  ║
╠════════════╬══════════════╣
║ user.id1   ║ category.id1 ║
║ user.id2   ║ category.id2 ║
╚════════════╩══════════════╝

SELECT COUNT(DISTINCT users.id) AS count FROM users
INNER JOIN ratings
ON users.id = receiver_id
LEFT JOIN ads
ON users.id = ads.creator_id
LEFT JOIN categories
ON categories.id = ads.category_id
WHERE user_type = "tutor"
AND categories.name IN ("economy","anatomy")