If $\mathrm{NO}$ reacts with $\left[\mathrm{Cr}{\left(\mathrm{CO}\right)}_6\right]$ how many $\mathrm{NO}$ are required to replace maximum number of $\mathrm{CO}$ group.

$\mathrm{CO}$ is $2$ electron donor and $\mathrm{NO}$ is $3$ electron donor,  $6 \mathrm{CO}$ are replaced by $4 \mathrm{NO}$.

In $\mathrm{Cr}{\left(\mathrm{CO}\right)}_6$, there are six CO ligands which are replaced by 4 NO ligands.