That is just an arbitrary number that you assign when you bind a transmitter to a receiver, you probably already have a receiver bound to the #0 position. I have all 3 of my quads receivers bound as #0, but I could have just as easily set each one up as a different number and created a new "model" to assign it to, but since I power on only 1 at a time I can do it the simple way. You would likely assign different numbers to different models inside the RC because they could have different setups conceivably, for instance you wouldn't want to accidently power on and bind to your airplane and go to fly if your model was actually set for your quad, the differences in model config could cause a crash. If that makes any sense.
Bottom line just pick one that is not in use and go for it.