I'd say about 2 feet apart.

You don't say if you want to make a hedge of these, if morning sunshine is a variety of Cordyline australis, then the spread of these is between 3 & 12 ft, so if you want to make a hedge, then I would suggest you plant them at least 4ft apart.
I wouldn't have thought these were a good subject for a hedge, as the leaves will always be at the top of the plant with just bare trunk below, Derek