Something

SELECT wp_pssst_competence.compete_ID, wp_pssst_competence.compete_name, 0 AS many
	FROM wp_pssst_competence
	WHERE wp_pssst_competence.compete_id
		NOT IN (SELECT wp_pssst_competence2users.compete_id FROM wp_pssst_competence2users)
	
	UNION
	
	SELECT wp_pssst_competence.compete_id, wp_pssst_competence.compete_name, COUNT(wp_pssst_competence2users.users_ID) AS many
	FROM wp_pssst_competence, wp_pssst_competence2users
	WHERE wp_pssst_competence.compete_id = wp_pssst_competence2users.compete_id 
	GROUP BY wp_pssst_competence2users.users_ID ORDER BY compete_id;