1. Each channel typically controls a single servo (or two servos using a Y cable). But moving of a stick can be used to control more than one channel/servo, using programmable mixes in the radio itself. That is one of the reasons why you have two sticks with typically 4 degrees of freedom, yet you can induce movement in more than 4 channels/servos. Some more discussion here :
http://www.rcindia.org/chatter-zone/no-wonder-some-people-need-12-and-14-channel-radios-!/
Another example. In a CCPM glow helicopter, when you push the throttle up, 4 servos move. One is the throttle servo, but 3 other "swash" servos also move with just that one stick movement.
2. Many radios support v-tail mixing. In your case for a quadrotor, you need 3 v-tail mixers (at least as per the design posted by Sai). The real problem is, you are doing mixing of signals that are coming out of the gyros etc, which is all happening in receiver land ! The v-tail mixing supported by radios, mixes stick positions on the radio, which is useless in your case (as there is no way to feed the output from your gyros etc back into the TX for doing the mixing there, and that too, only ONE mixer typically).