What is the Effective Atomic Number $(EAN)$ of Cobalt in $[Co(NH_3)_6]Cl_3$ (Atomic number of $Co = 27$)?

Among $V(CO)_6$,$Cr(CO)_5$,$Cu(CO)_3$,$Mn(CO)_5$,$Fe(CO)_5$,$[Co(CO)_3]^{3-}$,$[Cr(CO)_4]^{4-}$,and $Ir(CO)_3$,the total number of species isoelectronic with $Ni(CO)_4$ is. . . . . [Given atomic number : $V=23$,$Cr=24$,$Mn=25$,$Fe=26$,$Co=27$,$Ni=28$,$Cu=29$,$Ir=77$ ]

The $EAN$ of the central metal atom in $Fe_2(CO)_9$ is:

Which one of the following does not follow Sidgwick $EAN$ rule?

The Effective Atomic Number $(EAN)$ of $Co$ in $[Co(NH_3)_6]^{3+}$ is:

What is the $EAN$ of $Co$ in $[Co(NH_3)_6]^{3+}$?