Coordination number for polydentate ligands

[405972194]

How do we know what the coordination number for polydentate ligands will be? Such as a ligand like oxalate (C2H4) when drawing the lewis structure, it has lone pair electrons on all four oxygen atoms but why is it bidentate? What makes oxalate bidentate instead of oxalate as it has lones pairs on four atoms?

[JadaArreola_2E]

For polydentate ligands, we find the coordination number by looking at the donor atoms that can bind to a central atom. It is a bidentate because each molecule donates 2 pairs to the central atom. It is different from oxalate because of the planar structure which affects the oxalate bidentate because the shape allows coordination through the two atoms.