Find the number of left handed winners who participated in the WTA Championships.
SELECT count(DISTINCT winner_name) FROM matches WHERE tourney_name  =  'WTA Championships' AND winner_hand  =  'L'
SELECT count(DISTINCT winner_name) FROM matches WHERE tourney_name  =  'WTA Championships' AND winner_hand  =  'L'
[{'count(DISTINCT winner_name)': 2}]