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

, , and .

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