A big difference between modern warships and FS warships is in the guns. You don't need 20 guys per turret. There's no ammo to load (except missiles and flak, and those would have auto loaders anyway), there shouldn't be anything that you can manually maintain or adjust while it's firing. I think there would at maximum be 3 people per turret, and everything else automated.
How many total weapons on an Orion? 20? 30? x3 guys x3 shifts = 270 guys for weaponry. Say 30-60 command staff (including communications). 10-30 for navigation (assuming some really complicated stuff). 100-300 engineers that keep the reactors and vital systems running. 300-800 crew for various support duties, including food, light maintenance, cleaning, etc.
1460 estimated crew plus however many for damage control, marines, and flight crew.
What duties would damage control have on an Orion? I really have no idea, which is why I don't put numbers for that.
And remember: Just because you have room for 20 thousand folks, doesn't mean you need to carry that many.