How far can numbering go until it reaches the border of another?
For example the A320: It's derivatives are A318, A319, A321 and there could be but I don't know, a A322.
Now the A310's derivatives aren't numbered this way, there is no A309 or A311 (I think), so which plane could be an A315?
Boeing seem to have a somewhat convinent form of 7X7-xxx, there can be a 24th derivative of a 737 called 737-2400 if they really needed to, but Airbus is limited by it's choosing.
You see what I'm saying?