Number of ambidentate ligands among the following is _______.\( \mathrm{NO_2^-} \), \( \mathrm{SCN^-} \), \( \mathrm{C_2O_4^{2-}} \), \( \mathrm{NH_3} \), \( \mathrm{CN^-} \), \( \mathrm{SO_4^{2-}} \), \( \mathrm{H_2O} \).