Oxidation number of iron in ${\mathrm{Na}}_2\left[\mathrm{Fe}(\mathrm{CN}{)}_5\left({\mathrm{NO}}^{+}\right)\right]$ is:

Since ${\mathrm{Na}}_2\left[\mathrm{Fe}(\mathrm{CN}{)}_5\mathrm{NO}\right]$ contains nitrosylium ligand which is positively charged and represented as ${\mathrm{NO}}^{+}$. 

Let x be the oxidation number of irons in ${\mathrm{Na}}_2\left[\mathrm{Fe}(\mathrm{CN}{)}_5\mathrm{NO}\right]$

$\begin{array}{l}2(+1)+x+5(-1)+1=0\\ x=+2\end{array}$

Thus, the oxidation number of irons in ${\mathrm{Na}}_2\left[\mathrm{Fe}(\mathrm{CN}{)}_5\mathrm{NO}\right]$ is +2.