Far fewer than 9999 flight numbers are actually available.

9xxx flight numbers are not for regularly scheduled passenger flights (but may applied to a normal passenger flight that's been rescheduled in a way that the original flight number would have been ambiguous, for example flight AA1234 is delayed to the point where another flight AA1234 is also flying somewhere else in the system by that point).

Numbers above the first few thousand are reserved for regional jet flights and codeshare flights.

So there's only a few thousand flight numbers are available for AA's own metal mainline jet flights.