Number of ambidentate ligands among the following is ______
$$NO_2^-, SCN^-, C_2O_4^{2-}, NH_3, CN^-, SO_4^{2-}, H_2O$$

Ambidentate ligands are those that can coordinate through two different donor atoms.

$$NO_2^-$$: Can bind through N (nitro) or O (nitrito) — ambidentate ✓

$$SCN^-$$: Can bind through S (thiocyanato) or N (isothiocyanato) — ambidentate ✓

$$C_2O_4^{2-}$$: Bidentate ligand (binds through two O atoms of the same type) — not ambidentate ✗

$$NH_3$$: Monodentate, binds only through N — not ambidentate ✗

$$CN^-$$: Can bind through C (cyano) or N (isocyano) — ambidentate ✓

$$SO_4^{2-}$$: Not an ambidentate ligand ✗

$$H_2O$$: Monodentate, binds through O — not ambidentate ✗

Number of ambidentate ligands = 3 ($$NO_2^-, SCN^-, CN^-$$).

The answer is $$\boxed{3}$$.