For most planes up to .60 size one aileron servo will work fine (unless the plane is designed for dual servos). A couple of exceptions are when you need dual servos: my Alpha 60 had aileron flutter due to soft ailerons so I converted it to dual servos to eliminate the flutter, or if you want to convert to flapperons.

It is a fair amount of work to cut into a built-up wing and build up additional structure for dual servos if it wasn't designed for them in the first place.

Hogflyer