Bogatyr

SELECT cte1.cdate AS cd, 
       JSON_UNQUOTE(JSON_EXTRACT(cte1.rep, CONCAT('$[', cte2.num, ']'))) AS rep, 
       cte1.id AS did
FROM ( SELECT cdate, 
              id, 
              JSON_EXTRACT(jsonc,"$.Members[*].Number") AS rep
       FROM reports ) cte1, 
     ( SELECT help_keyword_id num
       FROM mysql.help_keyword ) cte2
HAVING LENGTH(rep) > 3