In the complex ion $[Ni(C_2O_4)_3]^{4-}$, the central nickel ion is coordinated to three oxalate ($C_2O_4^{2-}$) ligands. The oxalate ion is a didentate (bidentate) ligand, meaning it binds to the central metal atom through two donor oxygen atoms. Therefore, the coordination number of Ni, which is the total number of ligand donor atoms bonded directly to the metal, is $3 \times 2 = 6$.