In the non-carcinogenic molecules, how many contain chlorine atoms?
non-carcinogenic molecules refers to label = '-'; chlorine atoms refers to element = 'cl'
SELECT COUNT(DISTINCT T1.molecule_id) FROM molecule AS T1 INNER JOIN atom AS T2 ON T1.molecule_id = T2.molecule_id WHERE T2.element = 'cl' AND T1.label = '-'