That 10.000 looks nice on the plane, but has little value as a milestone. It certainly isn’t the 10.000th built (too many were skipped) and certainly also not the first with a 5 digit number. Many higher ones are flying already, at least up to 10.141 (N424AN, registered in April).
As of end August this year 9,496 A320 family planes have been delivered.
At Airbus an MSN number is an identity number in the order book rather than a sequence number on the FAL.
Cancelled orders will typically result in skipped MSN numbers.