Name the number of high rebounds for april 21
{'header': ['Game', 'Date', 'Team', 'Score', 'High points', 'High rebounds', 'High assists', 'Location Attendance', 'Series'], 'page_title': '2002–03 Boston Celtics season', 'page_id': '', 'types': ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text'], 'id': '1-17621978-11', 'section_title': 'Playoffs', 'caption': 'Playoffs', 'rows': [['1', 'April 19', '@ Indiana Pacers', 'W 103-100', 'P. Pierce (40)', 'P. Pierce (11)', 'P. Pierce (6)', 'Conseco Fieldhouse', '1-0'], ['2', 'April 21', '@ Indiana Pacers', 'L 77-89', 'A. Walker (19)', 'T. Battie (11)', 'P. Pierce (6)', 'Conseco Fieldhouse', '1-1'], ['3', 'April 24', 'Indiana Pacers', 'W 101-83', 'P. Pierce (21)', 'P. Pierce (12)', 'W. McCarty (6)', 'FleetCenter', '2-1'], ['4', 'April 26', 'Indiana Pacers', 'W 102-92', 'P. Pierce (37)', 'A. Walker (11)', 'P. Pierce (7)', 'FleetCenter', '3-1'], ['5', 'May 3', '@ Indiana Pacers', 'L 88-93', 'A. Walker (21)', 'E. Williams (8)', 'P. Pierce (8)', 'Conseco Fieldhouse', '3-2']], 'name': 'table_17621978_11'}