It appears as though RealFlight supports 12 channels, since you can map 12 input channels to 12 RealFlight channels. PPM supports at least 18 channels (like on the Futaba 18MA and Spektrum DX18). So I guess you could potentially program the radio's first 12 PPM channels as any of the outputs available on the radio, then use RealFlight to map those 12 input channels to any channel used by the model. If that's not the case, then the documentation should explain this a bit more clearly.
RealFlight also provides 4 virtual channels (5-8) so that you can use your keyboard if your radio has less than 8 channels. It doesn't appear to support virtual channels 9-12, not even through additional binding. So I guess if you have a 12-channel model, you can't operate channels 9-12 unless you have a 12-channel radio.