Coordination number, oxidation state, and the number of isomers in${\mathrm{K}}_3 [\mathrm{Cr}{({\mathrm{C}}_2{\mathrm{O}}_4 )}_3 ]$ are respectively.

Coordination number is the number of ligands attached to the central metal ion. ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$ is a bidentate ligand and in the complex, there are three bidentate ligands.

Therefore, the coordination number of Cr is 6. 

Oxidation state of Cr (x) is calculated as:

$$\begin{gathered} 3(\mathrm{oxidation} \mathrm{state} \mathrm{of} \mathrm{Kr})+\mathrm{x}+3(\mathrm{oxidation} \mathrm{state} \mathrm{of} {\mathrm{C}}_2{\mathrm{O}}_4^{2-})=0 \\ 3(+1)+\mathrm{x}+3(-2)=0 \\ \mathrm{x}=+3 \end{gathered}$$

Compound shows optical isomers: