Ambidentate ligands are ligands that can attach to a central metal atom through two different atoms. Let’s analyze the given ligands one by one:

• $$\mathrm{NO}_2^{-}$$: This ligand can bind through either the nitrogen (N) or the oxygen (O). Therefore, $$\mathrm{NO}_2^{-}$$ is an ambidentate ligand.
• $$\mathrm{SCN}^{-}$$: This ligand can bind through either the sulfur (S) or the nitrogen (N). Therefore, $$\mathrm{SCN}^{-}$$ is an ambidentate ligand.
• $$\mathrm{C}_2\mathrm{O}_4^{2-}$$ (oxalate): This is a bidentate ligand, but it coordinates through both oxygen atoms only. Therefore, it is not an ambidentate ligand.
• $$\mathrm{NH}_3$$: This ligand can only bind through nitrogen (N). Therefore, it is not an ambidentate ligand.
• $$\mathrm{CN}^{-}$$: This ligand can bind through either carbon (C) or nitrogen (N). Therefore, $$\mathrm{CN}^{-}$$ is an ambidentate ligand.
• $$\mathrm{SO}_4^{2-}$$ (sulfate): This ligand typically coordinates through oxygen atoms. It is not considered an ambidentate ligand.
• $$\mathrm{H}_2\mathrm{O}$$: This ligand can only bind through oxygen (O). Therefore, it is not an ambidentate ligand.

Based on this analysis, the ambidentate ligands are $$\mathrm{NO}_2^{-}$$, $$\mathrm{SCN}^{-}$$, and $$\mathrm{CN}^{-}$$.

Therefore, the number of ambidentate ligands among the given options is 3.