New:
SELECT tag.name, v1.number
  FROM (SELECT tag_id
             , COUNT(tag_id) AS number
          FROM tag_webobject     
         GROUP BY tag_id
       ) v1
  LEFT JOIN tag
    ON v1.tag_id=tag.id
;

Old:
SELECT tag.name, COUNT(tag_webobject.tag_id) AS number
FROM tag_webobject     
GROUP BY tag_webobject.tag_id
LEFT JOIN tag  
ON tag_webobject.tag_id=tag.id;