Each channel equate to a specific independent function. Multiple servos can be used to perform a single function.

example

2 servos connected to each airleron and plugged via a Y-harness to a single channel on the reciever will all a 4 channel or greater radio to utilize 5 servos but only for channels

The only exception I've seen to the above answer are cases where a modeler takes one channel or Function like full upelevator and mechinically couples an other to it like full up elevator actives a bomb drop or full throtle activates a smoke system I've never down it allways been lucky enough to have available channels on my radios