Key Idea The total number of electrons which the central metal ion appears to possess in the complex including those gained by it in bonding is called effective atomic number of central metal ion.
In the given complex ion $[Fe(CN) $$\left._{6}\right]^{3-}$, the $Fe$ is in $+3$ oxidation state. As we know that atomic number of iron is $26$ . The number of electron in $Fe ^{3+}$ is $23 .$ Each of the six cyanide molecules donates a pair of electrons so that EAN becomes $23+2 \times 6 \Rightarrow 35$