I have done both or just one.
While friction plates do wear, often they are still in spec and can be re used. But the kits seem to be a good price and not as high as when I had to replace them on my motorcycles.
So, it all depends on what you what to do. I dont recall the specs off the top of my head, the min spec for the plates.
If you have the manual handy, you can find those specs and then check them and if they are still good, re use them
But you can never go wrong with the whole kit.
Sporty