one's fine.
depending on what devices you use, one OR two might be better. i wouldn't worry about it. if you find you get "no more dsp connection" messages frequently, you can always add another. i'll bet you have no problems in the first place.
I love the synths (especially John Bowens) and use Scope mostly for them. Of course, I use the effects but not as much since I spent a mini fortune on UAD.