what is denticity of ambidentate ligand?

There are many ligands which have two different donor atoms in their structure. Such ligands can coordinate to the metal atom through either of their donor atoms and hence are given different names corresponding to the nature of the donor atom linked to the metal atom. Such ligands are known as ambidentate ligands. 

Denticity actually means the number of atoms in a ligand that can attach to the central metal atom / ion in a coordination complex. Ligands which are attached through only one donor atom are called monodentate, those attached through two donor atoms are called bidentate and so on. 

Denticity of ambidentate ligands refers to the ability of any ambidentate ligand to attach itself through different atoms in it. For e.g. ethylene diamine is a bidentate ligand as it attaches itself to the metal ion by its two nitrogen atom. However it can also act as a monodentate ligand if it is attached through only one of its nitrogen atom to the metal ion. Oxalate ion is also a bidentate ligand as it attaches itself through two oxygen atoms. EDTA is a polydentate ligand.