Solution 2 from JonA

New 2:
SELECT tag.name
     , COUNT(t1.tag_id) AS number
  FROM tag_webobject t1
  LEFT JOIN tag
    ON t1.tag_id=tag.id
 GROUP BY tag.id
        , tag.name
;

New 1:
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
;