Spare channel is in your TRANSMITTER not your receiver. So you want a 7 channel transmitter to have 6 servo plus the "spare" channel to serve as your flight mode control. That's why our NX6 has 7 channels (6 1/2 really).
Since you're using a DX9, you have 3 spare channels to choose from. That receiver is actually listening for 20 channels to come in, not just 6.
Andy