Gas turbines aren't big enough for that size ship, so you would have to clutch more than one per shaft.
Actually 2 turbine/shaft is fairly normal. The only reason to have 1/shaft is to allow twin prop redundency in smaller ships.
As for power the LM2500 can now put out 30,000 HP, with growth potential to 40K. Two of them coupled would supply as much power as you can put down a single shaft. An Iowa is going to need 4 shafts whatever the power source.