In the 18-electron structure of $\mathrm{Fe}{}_2(\mathrm{CO}){}_9$ (three bridging CO, each counted as one electron to each Fe, plus one Fe-Fe bond), the total number of terminal CO groups in the molecule is:

Each Fe has 8 valence electrons, 1 from the Fe-Fe bond and 3 from the three bridging $\mathrm{CO}$ (one electron each). That is 12, so $18-12=6$ more electrons are needed, i.e. 3 terminal $\mathrm{CO}$ per Fe. Total terminal $\mathrm{CO}$ = 6, plus 3 bridging = 9.