Not just the 555 but lots of rx have just the amount of plug ins as channels. Ex. a 5 channel rx only has 5 slots a 7 channel rx only has 7 slots.

Generally on the last slot it will say "channel 7/Batt". You just use a y to connect them. However, it does generally make more since to connect the ailerons with a y since you have to plug them in when you put the wing on. Its easier to have them plugged into a y harness and when you attach the wing you only have to plug in the harness and not each aileron seperately.