select count(*) as with_relations
  from (select id as ID, count(username) as count from j25_users group by username) as sub
  -- здесь нужно поменять user_object на твою таблицу, и, если нужно, UserID на нужную ссылку.
  inner join j25_users as tbl on tbl.username=sub.ID
  where sub.count > 1