It's actually rather obvious. The load on the ailerons of a 3D model is often considerable. It's actually common for some of them to experience blow-back when the servo(s) are not up to the load no matter how many. Look at the number of servos on most IMAC birds. Those big models often have multiple servos in each wing and more than one on the elevators and they're often rather huge servos to boot. Bottom line, there are a number of models that have 2-3 servos per aileron. So two per wing isn't even a limit. And we've not even discussed the aileron style, strip, barndoor, whatever. Nor what percentage of the wing area is present in the ailerons. Yeah, there are lots of planes that'd suck with just one aileron servo. What airplane is the one in question?

Trainers are usually 40-60 size and are designed to provide stable, rather sedate flight with no demand to execute quick, hard maneuvers. They're not powered to go fast and the flight envelope doesn't put the ailerons under appreciable load. You will see trainers with two aileron servos but it's obvious from the numbers that use one, that one is all that's needed. So the the airplane in question a trainer?

Is the airplane in question even a 40 or larger model? If it's a park flyer the wing might not even be large enough for two.

So there is a huge range to the loads that the huge range of ailerons we have will see. Yes, there are trainers with one servo, but can you think of any 3D birds that have just one? So there's the "proof".