I would say 2, for convenience sake.
I like to do every exercise with 2 dumbbells simultaneously, and that has worked very well for me, to build up muscle in a balanced between both sides of body.
There are 2 major exceptions (for me at least) however.
One is bicep curls, the other is wrist curls. Both of them, I can't imagine doing with both arms at the same time, because it's just so much better individually.
Another advantage of 2 dumbbells is that you can do lower body exercises equally - with one dumbbell in each arm.