How to count joined table?

SELECT t1.team_name,
       IFNULL(t2.num_players, 0) AS strength,
       t1.team_timestamp
FROM team t1
LEFT OUTER JOIN 
    (SELECT team_id, COUNT(team_id) AS num_players 
     FROM player 
     GROUP BY team_id
    ) t2
ON t1.team_id = t2.team_id
-- GROUP BY t1.team_id, t1.team_name, t1.season_id  -- **NOTE** - see discussion below
ORDER BY strength DESC, team_name ASC;