You need 2 channels for the ailerons, and also 2 channels for the flaps, if you want to be able to mix aileron input to the flaps. So, your layout would need 8 channels.
As said before big scale sailplanes can guzzle channels in a hurry.
I currently have a Multiplex mc 3030 with 9 channels, and am already reaching its limits ( channelwise), for example on my 4m ASH26 :
1ea. spoilers ( 2servos, but connected to one channel via Y-lead)
1 ea. tow-release
And this is not a really big scale sailplane ! There are scale sailplanes with inbd & outbd ailerons, flaps, spoilers, retract, rudder, elevator, tow-release.
Or even better : no tow-release, but an up&go electric powerplant, which requires another 2 channels, one for extension/retraction, and one for the speedcontrol.