Hey Shaun.My collection has a total of 57 locomotives. Both Steam and diesel. I have about 8 locomotives that are not powered, but will be soon. I find that an all powered locomotive roster leads to unlimited flexibility in making power assignments.
As for how many locomotives to have on hand. That is a matter of choice. Going by what is typical for a layout of your size, one is all that is really needed, but having around three is really nice. Any past that is gravy.
I model alot of things. But the only CSX trains I model is the Amtrak Auto Train and the Tropicana Juice Train.