Bigger airplanes can use 4 aileron servos......I would set mine up on dual channels with a Y to a pair of servos one on each wing and of course the other Y to the other pair of servos...that way if you lost a channel or a servo you still have some aileron left......
I have a bipe with 4 servos.....some of my flyin buds have 40 % monoplanes with 4 servos they don't have a problem with current...most of us are flying with Li-ion batteries and voltage regulators..or smart flys that allow 7.4 V for high voltage servos but 6V for the receiver...
As far as current draw...it won't be a steady draw but will vary with the load and amount of travel requested on the control surface